Sleep Inertia: An In-Depth Analysis

Introduction

Sleep inertia is a very common sleep phenomenon in humans and is defined as a period of impaired alertness upon awakening from sleep (including both core sleeps and naps)1. Whether the sleep pattern is monophasic or polyphasic, sleep inertia can occur in a seemingly random pattern; its degree of severity and effects also vary depending on different contributing factors. Concerns about how sleep inertia affects performance on different tasks (physical or cognitive) are also raised. There are a lot of questions to be answered about sleep inertia mechanics as well as any reliably absolute solutions that can effectively shut it down. Is sleep inertia an entirely negative aspect of sleep? Is it dangerous? Do homeostatic and circadian pressures have anything to do with it and is it wrong to live with mild levels of inertia on a daily basis? This blog post will serve to break down all related factors that can make sleep inertia worse or better, and whether it is entirely possible to avoid sleep inertia.

DISCLAIMER: Several findings of sleep inertia mentioned in the post may be contradictory from the past to the present day. More mechanics regarding sleep inertia are yet to be fully explored, and whether polyphasic sleep can completely handle sleep inertia over a sufficiently long period of entrainment to the sleep pattern cannot be confirmed currently. Therefore, more studies and baseline readings are required for extra comparison.

Factors causing sleep inertia

Prior Sleep duration:

This is considered a factor because of its relevance to homeostatic pressure. A common prediction would be that the higher the sleep pressure, the deeper the sleep becomes and the higher the chance sleep inertia is present (as the body is catching up with sleep). However, a polyphasic experiment showed that it was not simple to make such conclusions, at least in terms of non-reducing polyphasic schedules2. In an older study that experimented with the Tesla schedule (four 20 minute naps placed 6 hours equidistant from one another), the effect of sleep inertia was heavily amplified after some time had passed on the schedule. No participants overslept, which suggests that the level of sleepiness likely caused sleep inertia to linger for a much longer period of time than expected3. However, this does not reflect the full-blown adaptation process to polyphasic schedules unless the subjects stayed on the schedule long enough (at least 1 month). Tesla is also an extremely difficult schedule, if not outright unsustainable for the vast majority of the human population even short-term, while more realistic-looking polyphasic schedules with higher total sleep could likely yield very different results.

Prior sleep duration seems to be a reasonable factor that contributes to sleep inertia to a certain extent, however it is conclusively not a stand-alone factor with unknown mechanics to be explored.

Sleep Stages:

This factor appears to be one of the biggest. It was generally agreed that waking up in SWS gives the most sleep inertia, while REM wakes have varying degrees of sleep inertia and NREM1 and NREM2 wakes usually don’t cause noticeable problem-processing or solving skills moments after awakening1,2. REM sleep’s eye-movement density was found to determine whether the wake would cause a lot of sleep inertia or merely a moderate amount. High rapid eye-movement density as a result causes a lot more sleep inertia than low density.

Sleep Deprivation:

From the experiment with Tesla schedule, it is clear that intense sleep deprivation occurred after a short time on the schedule. Until adaptation is done, the body will not program automatic wake time to place light sleep at the end of each sleep, regardless of whether it is a nap or a core. Since the adaptation was never completed on Tesla, sleepers were constantly plagued with SWS/REM wakes. However, in one experiment, Claudio Stampi noted that performance upon awakening on a 3h polyphasic schedule did not show serious signs of impairment after at least a couple days. Sleep inertia was present, but affected different types of tasks1. Tasks such as Descending Subtraction (a task that requires counting down from 100 to psychologically check for cognitive functions) were severely hindered by sleep inertia, while tasks that required high levels of cognitive performance were not affected as much2.

When sleep deprivation level is moderate (partial sleep deprivation), it was also observed that in short naps (< 60 minutes), sleep stages play a dominant role in determining sleep inertia rather than sleep deprivation itself2. This is a reasonable conclusion – when first starting to sleep polyphasically, sleepers are advised to avoid having odd sleep lengths (e.g, 50 minute naps, 2h core sleeps) in their schedule to avoid SWS/REM wakes from the beginning.

Sleep Times in the Day:

Daytime naps were concluded to generate less sleep inertia than later naps in the day that are higher in the percentage of NREM sleep. Circadian-wise, later sleeps in the day approach the evening, which is closer to the SWS peak, so it is easier to incur SWS wakes and heavier sleep inertia. This explains why ideal Biphasic schedules do not favor late naps in the day (6 PM onward), not only because of potentially higher amount of inertia from the naps but also that night sleep can be delayed. Late naps on other polyphasic schedules are also discouraged, but the reason why some pure nap-only schedules have naps placed late in the day and at irregular hours is not fully understood.

It was noted that abnormal sleep-wake schedules (including certain polyphasic schedules that do not follow any specific rhythms) were proposed to cause more sleep inertia, especially if awakening occurs near circadian low hours (2-4 AM)2. However, this conclusion was based on the in-progress adaptation to polyphasic schedules, which is commonly associated with sleep inertia and bad wakes. The cognitive and physical states after adaptation is completed should have a very limited amount of sleep inertia or none in some cases. The vast majority of the polyphasic sleeping experiments weren’t long enough to conclude that polyphasic sleeping would always cause more sleep inertia upon awakening than monophasic sleep, and the majority of experiments were conducted with extreme nap-only schedules or very tough schedules with low success rate (e.g, Triphasic, 3 80 minute sleeps).     

Is sleep inertia dangerous?

Claudio Stampi remarked that sleep inertia can be bothersome at least and potentially dangerous at most at the wrong time1. During sustained operations where workers can only afford short naps around the clock, sleep inertia can affect the accuracy of the performance and lead to more human errors. Tasks that demand alertness right upon awakening also pose a big issue. Thus, it comes down to personal occupations to determine whether a polyphasic schedule with several naps or convoluted mechanics should be attempted, to avoid suffering from unnecessarily irritating sleep inertia.

Is it possible to avoid sleep inertia?

A very recent finding suggests that sleep inertia can be alleviated, as well as another niche means to drastically reduce its effects. Artificial dawn signals can be used to help minimize sleep inertia4. This includes the use of blue lights as a timer to stop melatonin secretion and start a new day. The use of artificial dawn also resulted in decreased skin temperature, a process associated with sleep. The reasoning was that during morning, core body temperature increases as the body wakes up, and artificial dawn promotes vasoconstriction of blood vessels to reduce the effect of arousals upon awakening. Another study suggested that the peak of alertness is not right after awakening – sleep inertia is always present and can last for minutes to hours; this implies that a small or negligible amount of sleep inertia does not pose any harm6.

An alternative to scheduling (under normal conditions and biphasic sleeping), is to make use of the Forbidden Zone of sleep. This zone includes a couple hours before night sleep (typically 19:00-21:00) with normal nocturnal sleep hours. By placing a nap right before the Forbidden Zone (but not inside the zone), it becomes possible to both get some sleep and wake up more easily with reduced sleep inertia effects. This is attributed to the body’s high alertness in preparation for the habitual night sleep and core body temperature is at the highest point in mid/late afternoon (4-6 PM). However, how the forbidden zone should be used to reduce sleep inertia as well as whether it is a reliable tool seems very inconsistent. Check out the Forbidden Zone blog post for more information.

Conclusion

Nowadays, lifestyles are filled with blue light, changes in sleep hours, a general lack of sleep hygiene practice and knowledge on sleep maintenance. Thus, it is understandable that a lot of humans are sleep deprived and wake up feeling unrested. Sleep inertia is very common and for the most part seems to be the norm of the current era. Minor sleep inertia of less than 15 minutes are considered normal and negligible1, but intensified sleep inertia is present under more extreme sleep deprivation conditions and possibly the improper usage of sleep duration (e.g, mid-cycle naps like 50 minutes). The ideal natural wake to aim for should be the light sleep stages to feel completely refreshed and free of sleep inertia. While it is usually not possible to completely avoid sleep inertia, efforts can be made to improve sleep quality to form consistent sleep routines. More research and data on the post-adaptation phase of polyphasic sleep is needed to conclude if sleep inertia is persistent or can be minimized with sufficient sleep hours and sleep hygiene.

Main author: GeneralNguyen
Page last updated: 12 May 2020

  1. Stampi, Claudio. Why We Nap : Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep. Birkhauser, 2014.
  2. Tassi, Patricia, and Alain Muzet. “Sleep Inertia.” Sleep Medicine Reviews. 2000;4(4):341–353. doi:10.1053/smrv.2000.0098. [PMC]
  3. Naitoh, P., et al. “Sleep Inertia: Best Time Not to Wake Up?” Chronobiology International. 1993;10(2):109–118. doi:10.1080/07420529309059699.[PMC]
  4. VAN DE WERKEN, MAAN, et al. “Effects of Artificial Dawn on Sleep Inertia, Skin Temperature, and the Awakening Cortisol Response.” Journal of Sleep Research.2010;19(3):425–435.doi:10.1111/j.1365-2869.2010.00828.x. [PMC]
  5. Lavie, P., and B. Weler. “Timing of Naps: Effects on Post-Nap Sleepiness Levels.” Electroencephalography and Clinical Neurophysiology. 1989;72(3):218–224. doi:10.1016/0013-4694(89)90246-0. [PMC]
  6. Krauchi, K., Cajochen, C., & Wirz-Justice, A. Waking up properly: is there a role of thermoregulation in sleep inertia? Journal of Sleep Research. 2004;13(2):121–127. doi:10.1111/j.1365-2869.2004.00398.x [PMC]

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