This article is a guest piece from Brandon Roberts (PhD). Brandon is currently researching muscle hypertrophy and has a specific interest in the role of ribosome biogenesis during resistance training. He also writes science-based content for SCI-FIT, AARR, StrongerbyScience, and various other websites. You can find links to his articles here and links to his scientific publications here.
TABLE OF CONTENTS
2. Why do we sleep?
4. Sleep & Performance
- Study #1 – Bodybuilders
- Study #2 – Powerlifters
- Study #3 – Weightlifters
- Study #4, #5, and #6 – General Strength
- Study #7 – Motor Learning
- Response Heterogeneity
- What else can play a role for athletes?
6. Sleep & Nutrition
7. Objective Methods to Measure Sleep
- Sleep Tracking
8. Subjective Methods to Measure Sleep
- Sleep Diaries
- Pittsburgh Sleep Quality Index (PSQI)
- Sleep Hygiene Index (SHI)
- Morning-Eveningness Questionnaire (MEQ)
- Epworth Sleep Scale (ESS)
- Athlete Sleep Behavior Questionnaire (ASBQ)
9. How can we improve sleep?
10. Sleep Extension
- Study #1 – Collegiate Athletes
- Study #2 & #3 – Professional Athletes
- Sleep Education
- Sleep Hygiene
11. A Guide to Improving your Sleep
12. Sleep Goals
- Key Terms
- Can we recover from a bad night of sleep?
- Will naps help me recover from a lack of sleep?
- How does sleep quality play a role?
- What is sleep debt and can it be repaid?
A common attitude in our culture is that the ability to tolerate insufficient sleep is a strength; sometimes even a badge of honor (Adler 2009). Yet, we know sleep is important. We also know that we need around eight hours per night, which falls in line with the National Sleep Foundation’s recommendations (7-9h) (Hirshkowitz 2015). What you may not know is how sleep affects performance or day-to-day decisions. In this article we’ll go through a number of studies – some in athletes and lifters – to help you understand why sleep is important, methods of tracking it, and what happens when you don’t have enough of it. We’ll also take a brief foray into chronotypes, response heterogeneity, nutrition, and how they can affect or be affected by sleep.
2. Why do we sleep?
Scientists are still debating the function of sleep. Here are some of the main hypotheses (Vyazovskiy 2015):
- Sleep restores the immune and endocrine systems
- Sleep helps the nervous and metabolic systems recover
- Sleep has an integral role in learning and memory
It’s probably a combination of all of these.
Disclaimer: I do not go into detail on stages of sleep (Stage 1-4, NREM, REM) as it would make this article much longer than necessary.
It is estimated that more than one-third of Americans are not meeting the sleep recommendations (Liu 2016) – with people over age 30 reporting the shortest sleep duration (Ram 2010). It is also estimated that 5 – 15% of Americans suffer from difficulty falling or staying asleep (Ohayon 2002), while one-third report waking up multiple times per night (Guilleminault 1988). These sleep disturbances ultimately reduce sleep quality, sleep efficiency and perceived sleep quality (Guilleminault 1988).
Why does that matter? Well, a decrease in sleep quality can result in sleepiness and dysfunction during the day. At a general health level, daytime sleepiness from a night of impaired sleep can lead to problems with judgment, focus, productivity, and can even elevate the risk for vehicular accidents (Rosekind 2010).
For the college student it might matter even more since reduced sleep can impair academic success. One study reported 31% of students suffered from morning tiredness (Buboltz 2011). Another suggests that poor sleepers have reduced daytime function (Alapin 2004). A common solution is to grab a stimulant to offset sleepiness, but we wouldn’t want to make that a long-term habit because it can cause anxiety or even interact with some medications (Seifert 2011). Also, the cost of regularly drinking stimulants adds up quickly.
So let’s say you stayed up all night to cram for an exam. That might help you in the short term, but overall sleep duration and irregular sleep schedule correlate with a lower GPA (Gaultney 2010). Drinking alcohol could also have an effect. For example, a number of studies have shown that drinking can cause acute sleepiness, but can also result in frequent awakenings (Ebrahim 2013). I don’t think I’ll be able to talk anyone into giving up alcohol, so we’ll leave that for another article.
4. Sleep & Performance
Now that you have some background knowledge on sleep let’s look at a few studies in athletes.
It wouldn’t surprise anyone that exercise improves sleep. A systematic review of 21 studies found that people with higher levels of physical activity experience better sleep (Lang 2016). However, physical activity encompasses a ton of different types of exercise. What if we only look at athletes? Well, elite athletes have a prevalence of sleep disturbance ranging from 15 – 70% (Gupta 2016). However, most people aren’t elite athletes. Let’s take a look at a few studies that could apply to a trained yet not elite population.
Study #1 – Bodybuilders
In my work with natural bodybuilders (and as one), I’ve often heard athletes report a disruption in sleep patterns during contest preparation. The lack of sleep is often attributed to the drive to find food via evolutionary mechanisms. Sometimes this extra time awake can be good, like when I use it to propel me through writing manuscripts or articles. Sometimes it can be bad, like when it affects training performance or mood. In a case-study on a bodybuilder, sleep was measured over a 13 month period. Sleep efficiency was reduced by a maximum of 3.6% over the pre and post-competition period. Interestingly, sleep duration increased 20% and sleep efficiency increased 8.9% in the contest preparation phase. This would indicate that the bodybuilder slept more, which means he’s not eating and not hungry. Sounds like a win. However, this subject had poor sleep habits (Pardue 2017). He was sleeping less than 6 hours per day at the start of the study.
Study #2 – Powerlifters
Much like bodybuilding, there isn’t a lot of research on sleep in powerlifters. In a study on injuries in a group of German powerlifters, good sleep was correlated with fewer injuries, although no further objective analysis of sleeping parameters like duration and quality was measured (Reichel 2018). As we know, correlation isn’t causation, but it does give a little bit of confirmation bias: sleep is beneficial.
Study #3 – Weightlifters
There aren’t many studies on sleep in weightlifters either. In collegiate weightlifters, there were no differences in performance following 24 hours of sleep deprivation compared to a normal night of sleep in a single lifting session (Blumert 2007). However, there were negative changes in mood resulting in more fatigue and overall mood disturbance. This study hints that we might be able to have one bad night of sleep without acute performance issues.
Study #4, #5, #6 – General Strength
General strength could also be affected by a more prolonged lack of sleep. While a bit extreme –a study of partial sleep deprivation using 3 hours of sleep for 3 successive nights found decreased performance in the bench press, leg press, and deadlift after the second night of sleep loss (Reilly 1994).
Another study used six days of intense training where subjects trained twice per day (morning and afternoon) with either HIT or strength training. The subjects reported increases in fatigue, emotional exhaustion, physical complaints and decreases in sleep quality as part of a questionnaire they were given during the study. Oddly, objective measures of sleep weren’t changed (Kolling 2015). The subjects slept slightly less than 7 hours per night with no decrease in total sleep time or time in bed. This is the first indication that objective and subjective sleep can be different. We’ll visit how we measure both of those later.
The longest sleep deprivation study I found lasted 60 hours, yet found no effect on maximal upper and lower body isometric and isokinetic strength tests (Symons 1988). However, neither of these are very translatable to the everyday lifter.
Study #7 – Motor Learning
Lifting technique, driven by motor learning can be affected by sleep too. A ton of research has demonstrated that sleep is able to enhance cognitive and motor tasks (for a review see Christova 2018). For example, one study found sleep results in a 20% increase in motor speed without losing any accuracy (Walker 2002). So maybe next time you’re refining your technique get a good night of rest.
Response heterogeneity is something the research world is starting to see as really important. It exists in the sleep literature too. For example, in the Blumert study some people had large deficits due to sleep deprivation, some had none, and some participants improved performance. That study didn’t control everything so there could be other factors involved like nutrition or stress. One way the sleep literature has looked at response heterogeneity is by grouping people by chronotype which I’ll discuss in the next section.
What else can play a role for athletes?
- Seasonal phases: Sleeping patterns can change through the year due to sport schedules. Some team-sport athletes sleep longer in the off-season compared to the in-season (Swinbourne 2016). Of course, bodybuilders and powerlifters don’t have normal seasons like most athletes so it may not translate directly to them.
- Sleep environment: Where you sleep, especially before a competition, can contribute to sleep impairment. For example, there is a “first-night effect” which occurs due to a new sleep environment (Suetsugi 2007). The main effects are lower time sleeping and decreased sleep quality. So, if you’re competing somewhere outside the range of your own bed you may want to travel to get there a few days early.
There is some data to suggest certain people can survive on less sleep. These people have a subvariant of Gene DEC2 (P384R) and are known as the sleepless elite (He 2009). This variant was identified in a family, and carriers routinely sleep ~1.5 hr less than non‐carriers. Previous studies from the 1930s found a better agreement of sleep between identical twins compared to fraternal twins, meaning that sleep has a genetic component (Dauvilliers 2005). Lack of sleep can also be fatal, as demonstrated by the fatal familial insomnia. There’s very little research on how to identify if you’re able to survive on less sleep, so I urge those reading to assume they are the norm rather than the exception.
Circadian typology, also known as chronotype, categorizes people as morning, evening, or neither/intermediate with respect to their preference for activity and sleep. Chronotype is not a novel concept – it dates back to the 1940s (Kleitman 1949). Although during the past decade chronotypes have become more popular in research (Adan 2009). Most people are an intermediate chronotype and ~15% are categorized as strictly morning or evening types (Rosenthal 2001). Women and older adults have a strong tendency to be morning types (Adan 2010).
Athletes are commonly morning types, but some data suggests people select and participate in sports that match their chronotype (Kunorozva 2012, Rae 2015). Although athletes can adapt to early morning training, there is speculation that athletes involved in morning sports who are evening types may not have progressed from sub-elite or amateur levels because of their training time (Lastella 2016). I don’t necessarily think that matters a whole lot.
The time of day we exercise could be important for performance. One hypothesis is body temperature has an effect on muscle contractile properties (Bergh 1999; Martin 1999). Another is that performance differences are due to circadian body temperature variation which elevates later in the day. In fact, one study found a ~5% difference in vertical jump performance between morning and afternoon testing if an extended warm-up was included (Taylor 2011). So, if we do train in the morning it’s important to sufficiently warm-up.
Wake-up time can also be a predictor of peak performance and varies across chronotype (Vitale 2015). One study found that time since awakening is the major predictor of peak performance times – not time of day – and could explain variations in performance of ~25% over the course of the day (Facer-Childs 2015). It also appears that evening types may need more time to prepare for training after waking up, which makes sense from a practical perspective. Some athletes can roll out of bed ready and others need a few hours to feel ready to compete.
To conclude: I’m not sure we have enough data on physique, power, or any athletes to use chronotype yet. One of the main findings is chronotype can influence RPE with morning types performing better in the morning, as one might expect (Vitale 2017). It might be a good idea to train at the same time of days as your competition, if possible (Chtoura 2005). For example, powerlifters often lift much earlier or later during a competition than their normal training time.
6. Sleep & Nutrition
I would be remiss to not include how sleep, performance, and nutrition interact. In general, studies have shown that those who sleep <7h per night have higher energy intake from snacks and high fat foods, especially teenagers (Weiss 2010). Lack of sleep is associated with decreases in leptin, increases in ghrelin which increase overall hunger, and appetite for calorie-dense foods (Spiegel 2004). There is also a tendency to increase snack intake during sleep restriction (Nedeltcheva 2009) – with fat being the macronutrient of choice (St. Onge 2011).
Reduced sleep can have an effect on energy intake depending on the person (McNeil 2017, Calvin 2013). One study found large inter-individual variations in response to sleep restrictions, whereby some participants increased by ~1000kcal and other decreased by ~1000kcal. Another study found similar results with energy intake with large differences in body weight changes from -5 to +13lbs after 5 nights of sleep deprivation (Spaeth 2015). In practice, it would be beneficial to know if you or your athletes change eating patterns when sleep restricted so you can prepare accordingly. For example, you may need to rearrange food intake around a certain day if you know you will have reduced sleep because you need to study for an exam or work on a big project.
Energy substrate balance appears vulnerable to sleep loss, with 30 hours of sleep deprivation reducing muscle glycogen when combined with an intermittent sprinting protocol (Skein 2011). Reduced glycogen could translate to a reduced work capacity or early onset of fatigue during a workout. There appears there are no RPE related increases with light cardio, which isn’t surprising (Martin 1982). Yet, lack of sleep can decrease physical activity during the day, which could play a role in daily nutritional needs (Schmid 2009).
7. Objective Methods to Measure Sleep
Now, let’s look at some tools we can use to help determine our sleep patterns.
Polysomnography (PSG) is considered the gold standard to objectively assess sleep. PSG uses electrodes that monitor brain activity, eye movements, heart and muscle activity. It can also measure different stages of sleep. In other words, participants have to be hooked up to a machine while they sleep. The most important aspect of polysomnography is that all other measures of sleep are validated against it. As you can imagine, this method isn’t very practical for most people nor is it really reflective of what happens ecologically since it is measured in a very controlled environment. It’s akin to a metabolic ward in nutrition studies.
Since most people don’t have access to polysomnography (PSG), the first thing that usually comes to mind is a smart device (e.g., Apple Watch™). Most of these devices work using actigraphy (measures movement). Actigraphy has become a widely used tool, as it has been found to be much more reliable than subjective or self-reported sleep diaries and behavior logs (Arora 2013). Not everyone agrees though because a review in 2015 found that accelerometers are accurate for steps but not necessarily for energy expenditure or sleep (Evenson 2015).
There are two validated devices against PSG. They are the Fitbit Charge 2™ and the Oura ring (da Zambotti 2017, da Zambotti 2018). I haven’t used either of them and personally use my Apple Watch™ with the Pillow app. In general I don’t think many devices are accurate in measuring stages of sleep based on the literature (Mantua 2016) but most devices are good enough to measure total sleep time, even if it’s crude. If you’re concerned about accuracy then it’s best to check and make sure the data makes sense each morning. I think of it in these terms: if my goal is to sleep ~8 hours per night and my devices tells me I’ve increased from 7 to 8h, then I’m going to consider it a success. Sometimes we get too caught up in the science of things.
8. Subjective Methods to Measure Sleep
There are several methods to measure subjective sleep. In this section we’ll go through a few that are often used in the literature.
Sleep diaries are one of the easiest ways to track sleep. All you have to do is answer a few questions after you wake-up.
Sleep diaries generally include:
- Time of getting into bed
- Time you attempted to fall asleep
- Sleep onset latency
- Number of awakenings
- Duration of awakenings
- Time of final awakening
- Time you got out of bed
- Sleep quality
- Space for other comments.
This method is cheap, quick, and allows some personal customization of questions. As you could guess, most of the limitations are due to misreporting. For example, one study reported sleep an hour earlier than actual time (Carney 2004) while another found sleep diaries overestimate nap frequency, duration, and sleep latency (Lockley 1999).
Here’s an example from the Consensus Sleep Diary.
Pittsburgh Sleep Quality Index (PSQI)
The Pittsburgh Sleep Quality Index (PSQI) is one of the oldest sleep questionnaires around. It is often used in clinical research and includes a 19-item, self-rating tool. It assesses sleep quality and discriminates between “good” and “poor sleepers” (Buysse 1989, Dietch 2016). The PSQI has been shown to have a high degree of internal consistency and has been validated against clinical and laboratory diagnosis of “good” and “poor” sleepers but is not without limits. If 19 questions seems like too much, there is even a short version called the shortPSQI which is only 13 questions and gives similar data (Famodu 2018).
What’s the shortPSQI look like? Here’s a Google doc so you can take it yourself.
Sleep Hygiene Index (SHI)
The Sleep Hygiene Index (SHI) measures wake behaviors that may adversely affect sleep; it has 13 questions rated on a scale of 1 (never) to 5 (always) with total scores ranging from 13 to 65 (Mastin 2006). Sleep hygiene relates to sleep routine, stimulus-control, health, environmental, and cognitive variables that impact the quality and quantity of sleep.
Want to take the SHI? Use this link to take it yourself.
Morning-Eveningness Questionnaire (MEQ)
The Morning-Eveningness Questionnaire (MEQ) is a self-assessment based on your preferences of performing activities in the morning or evening (e.g., if you had no time commitment what is your preferred wake-up time?). The questionnaire has 19 questions with a final score ranging from 16 to 86 (Horne & Ostberg 1976). Five categories indicate circadian preference and range from definitely morning type (70–86), moderately morning type (59–69), neither type (42–58), moderately evening type (31–41), to definitely evening type (16–30). This test is used to determine chronotype.
You can take an automated version of the MEQ here.
Epworth Sleep Scale (ESS)
The Epworth Sleep Scale (ESS) is used to assess daytime sleepiness (Johns 1991). This survey asks you to rate the probability of falling asleep in eight different situations (e.g. sitting and reading) on a scale ranging from 0 (no chance) to 3 (high chance). The scores for the eight questions are added to obtain a single number. A score between 0–9 is considered to be normal while a score of 10–24 indicates excessive sleepiness.
You can take an automated version of the survey here.
Athlete Sleep Behavior Questionnaire (ASBQ)
The Athlete Sleep Behavior Questionnaire (ASBQ) can differentiate the sleep practices between athletes and non-athletes. The ASBQ will also provide information where improvements can be made (Driller 2018). A pilot study using the ABSQ was completed in elite cricket athletes (Driller 2019). The authors found that individualized sleep hygiene plans led to improvements in sleep efficiency, latency, and sleep onset variance.
9. How can we improve sleep?
It’s not enough to know a lot about sleep if we don’t use it to help improve our sleep. There are some basic guidelines at the end of this article, but I want to include the two most prominent ways to improve sleep.
Study #1 & #2 – Collegiate Athletes
As little surprise to anyone, It appears that increasing sleep on purpose can be beneficial for athletes. This practice is called sleep extension. A study on basketball players found an increase in sleep of 2 hours per night resulted in better performance (Mah 2011). These players had a faster sprint time (+5%), better shooting accuracy (+9%) as well as more vigor and decreased fatigue. Maybe even more importantly, they reported better ratings of physical and mental well-being during practices and games.
Another study used sleep extension in tennis players (Schwartz 2015). The authors found that 7 days of sleep extension, where athletes increased sleep time from ~7 hours to ~9 hours, resulted in an increase in serve accuracy. In combination, these two studies show that college athletes, who have to juggle sports and academics, could increase performance with more sleep.
Study #3 & #4 – Professional Athletes
Much like collegiate athletes, pro athletes have sleep issues which can be helped by extended sleep. A study using sleep extension on pro rugby players found it could increase sleep time and sleep quality as well as decrease stress and improve reaction time (Swinbourne 2018).
Another study using a six week sleep optimization program in Australian soccer players found improvements in self-reported total sleep time (+20min), sleep efficiency (+2%), fatigue and vigor. Yet, the actigraphy data suggested no improvements in sleep time or efficiency (Van Ryswk 2017). The sleep education consisted of a one hour information session during the first week and midway through the study. A novel idea this study used was feedback. Participants were updated once a week with their data and positive benefits of improved sleep based on the current research. Plus they were offered sleep consultations after the study midpoint. This could be a very practical way for coaches and/or teams to help athletes manage sleep.
Ultimately, there are a number of studies on athletes which almost all indicate that high level athletes sleep less and have lower quality sleep than the general population even though they need more. Sleep extension efforts will likely pay off for some of these athletes.
Sleep education can be an important factor to improving sleep. It’s part of why I wrote this article. My sleep actually improved drastically because I became aware of all the benefits of sleep. As I mentioned in the intro – we already know we need sleep, so what’s the point in education? Well, maybe it’s just being reminded that sleep is important. In a time when we have tons of distractions – maybe we just need a gentle reminder to take care of ourselves.
There is even data suggesting sleep education is important. One study used a brief online and personalized sleep educational website, Sleep to Stay Awake, which improved sleep knowledge and resulted in improved sleep behaviors, sleep quality, and decreased depression scores in those using it. The improvements occurred at about week 8 of the study (Hershner 2018). Another study piloted a sleep education program in high school students, but it appeared to only increase sleep on the weekends (Kira 2014). Finally, a systematic review on the effects of sleep education found a discrepancy between sleep hygiene knowledge and sleep education (Dietrich 2016). Oddly, one study in the review found no differences in knowledge post-sleep education even though the subjects’ behaviors were better (Brown 2008). Taken together, this means that sleep education could work for some people but not others. It’s definitely worth a try if you have sleep issues.
The Sleep to Stay Awake website can be found here.
Sleep hygiene are practices that help sleep quality and avoid behaviors that interfere with sleep. Education on sleep hygiene generally includes various lifestyle modifications such as light, temperature, and noise. For example, one study found improvements in sleep time and decreases in the number of times participants woke up using an education session (O’Donnell 2017) that focused on the following practical tips for good sleep hygiene:
- Maintain regular bed and wake times
- Sleep in a quiet, cool, and dark environment
- Avoid stimulants prior to sleep
- Avoid LED prior to sleep
- Use relaxation strategies before bed
10. A Guide to Improving your Sleep
Now that you have some tools to use, how do you implement them?
- Take the PSQI, ASBQ, and SHI to determine if you have sleep problems. If issues are present try to identify how, when, and why sleep issues occur.
- Use an objective measure to track your sleep for a week. Compare this to your subjective measures. Avoid early morning training sessions following sleep disruption or deprivation where possible.
2. Take the MEQ to determine if you have a specific chronotype. You may not be able to adjust anything in your life, nor will the results of this survey likely surprise you. However, if you can adjust life try to shift training towards your chronotype. This research area is very novel so it may not make a difference.
3. Re-read this guide next week, then next month, then in three more months. Understand what a poor night of sleep can do to you. Know that it is more likely to affect long term performance through injury and illness. Sleep education…remember?
4. Create goals to improve specific aspects of sleep. Focus on one goal at a time (see below for goals).
5. Where possible, align training sessions to competition time to adjust for circadian rhythms. However, such practices have logistical issues and should not be at the risk of the quality of training.
6. Coaches and athletes should supplement the understanding of sleep loss and performance with an increased knowledge of the relationship between sleep and recovery – much like focusing on food as fuel we need to focus on sleep as recovery.
11. Sleep Goals
- Maintain a regular sleep/wake cycle
- Aim for 8-9 hours of sleep per night. Account for the time you spend in bed not
- Limit caffeine intake in the afternoon
- Limit alcohol intake
- Sleep in a cool, dry environment with minimal outside light
- Limit technology use 1-2 hours before bed
- Reduce ambient light in the late evening hours
- Create a sleep ritual to allow for a 30-60 minute wind-down period before bed
12. Key Terms
Sleep restriction: When we fall asleep later or wake up earlier than normal.
Sleep deprivation: Extreme cases of sleep loss, whereby we do not sleep at all for a prolonged period.
Sleep efficiency: The percentage of time spent asleep while in bed.
Formula = Minutes asleep / minutes in bed
Sleep latency: The time it takes to fall asleep once we are in bed.
Total Sleep Time: The amount of hours we sleep each night.
Sleep hygiene: a variety of different practices that are necessary to have normal, quality nighttime sleep and full daytime alertness (NSF).
Sleep debt: The cumulative effect of not getting enough sleep.
I queried one of the larger Facebook groups on questions related to sleep and performance, here are some of their questions not addressed directly in the article:
Can we recover from a bad night of sleep?
Sleep extension can help remedy a bad night of sleep. However, in one study subjects who had potential sleep debt needed four days to recover (Kitamura 2016). Therefore, you can recover from a bad night, but it may take more than one night of sleep (Sallinen 2008). See below how naps can help (Gillberg 1996).
Will naps help me recover from a lack of sleep?
Napping can be beneficial for those with sleeping disorders or shift workers (Takashi 2003). Furthermore, naps can help daytime sleepiness in some people (nappers) but not others (non-nappers). One study found no changes in cognitive or behavioral performance with a nap intervention of 4-weeks (McDevitt 2018). The main issue for most people is sleep inertia, which is the grogginess you feel when you wake up from a nap.
Some people take naps in response or to prepare for sleep debt. Some people do it for fun or out of boredom. Even for individuals who generally get the sleep they need on a nightly basis, napping may lead to considerable benefits in terms of mood, alertness, and cognitive performance. Practically, if naps do not interrupt your normal sleep patterns they can be beneficial; however, if you can’t sleep because you had a nap then it’s probably not a good idea.
How does sleep quality play a role?
- Sleeping at least 85 percent of the total time in bed
- Falling asleep in 30 minutes or less
- Waking up no more than once per night; and
- Being awake for 20 minutes or less after initially falling asleep.
A survey in 2014 by the National Sleep Foundation found that 35% of American adults rated their sleep quality as “poor” or “fair”. Luckily, exercise can improve sleep quality (Banno 2018). Mindful meditation may also improve sleep (Rusch 2019).
What is sleep debt and can it be repaid?
Sleep loss that accumulates after having reduced sleep for consecutive days is defined as sleep debt. A dose-response study of sleep doses of 4, 6, or 8 hours of (Van Dongen 2003) for 14 days found that performance, memory and cognition were adversely affected. In fact, chronic sleep of 6 hours or less had cognitive deficits that were similar to ~2 nights of total sleep deprivation. The most interesting part of this study is that subjects didn’t realize the increasing cognitive losses, which means you wouldn’t even know if you were sleep deprived. Therefore, even if you don’t feel tired can still have sleep debt, which is called potential sleep debt.