What is sleep inertia?
Sleep inertia is the feeling of grogginess that may follow sleep. There are different types of sleep inertia and there is a monstrous confusion in terminology, as well as a great deal of confusion between different types of sleep inertia in scientific literature. An example of a confusing definition of sleep inertia: "Sleep inertia refers to the feeling of grogginess most people experience after awakening". A more appropriate definition would say "Sleep inertia refers to the feeling of grogginess that is a result of interrupted sleep or other violations of sleep hygiene". Most of all, sleep inertia is not an inevitable part of sleep in humans. In healthy individuals, sleep inertia is a direct result of errors in the art of sleeping. With a religious adherence to the principles of sleep hygiene, you need not ever experience sleep inertia and its negative consequences for learning, attention, health, etc.
All research into sleep inertia should clearly distinguish between its different types:
- interrupted deep sleep - in the exemplary hypnogram, we can see Stage 4 NREM setting in after some 30 min. of sleep. Waking up a sleeping subject at this stage is particularly difficult. When woken from deep sleep, we experience an overwhelming need to get back to sleep. The feeling can be compared to being hit on the head with a heavy object. The brain is in a state entirely unsuitable for processing waking information. This is the torture that many polyphasic sleepers impose on themselves by trying to interrupt "naps" taken in their subjective night phase.
- interrupted REM sleep - in the hypnogram, REM sleep is marked in blue and its occurrence increases towards the end of night sleep. Interrupting fully-blown REM sleep is equally unpleasant. Yet the neurohormonal state of the brain is entirely different than that when the deep sleep is interrupted. One of the easily recognizable hallmarks is muscle weakness. Due to the function of REM, the motor system is turned off in this phase. If you were to test your strength on a hand gripper, you might score less than your kids. Interrupted REM is also frequently associated with dream recall. As soon as the brain returns to its typical waking mode, REM dream memories dissipate very fast. If you do not write down your dream instantly, it may be unrecoverable from memory as soon as 5 min. later.
- waking in a wrong phase - even natural waking can result in sleep inertia. If you wake up naturally in the period of your subjective night sleep, you may feel pretty groggy for a while. The main cause of premature waking is an early bedtime. Sleeping in an early phase is pretty widespread. Many people need to wake up early against their body clock. They will often use various remedies to fall asleep early (from alcohol to sleeping pills). As a result, they will fall asleep early, wake up early, and seemingly get "sufficient" sleep while still feeling tired and unrefreshed. This is because they wake up while their body is still, in circadian terms, in the period of the subjective night. Early bedtime will often result in insomnia. However, when sleep is initiated successfully, the sleep control system can launch an equivalent of night sleep ahead of time. Such sleep may run its course and even last a bit longer. However, its structure will differ, and the morning circadian sleep propensity will still not be fully cleared on waking. As a result, morning grogginess will result as a combination of circadian sleepiness and various sleep deficits caused by a change in sleep structure (e.g. REM deficit). Inertia caused by early waking is far easier to combat than interrupted NREM or REM sleep as it largely dissipates with the expiration of the circadian sleep propensity. However, minor sleep variable deficits may last for the course of the day. For some people, sleeping in a wrong phase is so much of a daily reality that they tend to forget what a crisply alert mind is, and, as in the mis-definition quoted above, they tend to think this is the "type of sleep inertia most people experience", as if it was part of normal human physiology. They might dismiss it by saying "I am just perpetually tired. It is just me". Technically, the definition of sleep inertia should also be extended to the brain state caused by a major delay in bedtime. That brain state is similar to the inertia caused by early waking. It may feel more natural as it combines both components of sleep propensity: homeostatic and circadian sleepiness. Instead of feeling like inertia, it will feel like severe sleepiness that might verge onto nausea. That type of inertia is particularly dangerous for drivers as it only gets worse in time and may results in moments of microsleep when portions of the brain cortex simply enter the sleep mode with the appearance of theta waves in the EEG.
Does sleep inertia show a circadian rhythm?
This question does not have a straight answer. Whatever you read on the subject, make sure you deconvolve the all-encompassing term "sleep inertia" and ask the same question for each of the types of sleep inertia. If you interrupt deep sleep, it will always feel bad. The degree of that feeling will likely depend on the depth of sleep, your homeostatic status and, to a lesser degree, your circadian status (only because deep sleep is largely homeostatic). However, if you interrupt REM sleep, it is more likely to have a more profound effect at the times of the circadian REM peak. Finally, the wrong-phase inertia is purely circadian. It will hit you only in the periods of your subjective night, and it will dissipate on its own at time of your subjective day.
How can I recover from sleep inertia?
You can google out dozens of remedies against sleep inertia (example), and you might be amazed that there is a big wide hole in reasoning behind all that "Internet advice", which often fails to notice that: well-timed sleep is the best remedy against all forms of sleep inertia!
For interrupted sleep inertia, NREM or REM, the simple remedy is: go back to sleep. The more powerful the inertia, the greater your chances of quickly falling back asleep. Remedies like coffee or exercise might make you feel better (or not), but they can do their own damage. If your profession calls for waking up in the middle of the night, remember that you are doing the service at the cost of your own health and longevity.
Wrong-phase inertia is a bit harder to combat. In many cases you won't be able to fall asleep. Even worse, trying to sleep can sometimes make things worse. The best solution is to suffer through the discomfort, avoid napping till your next subjective night period, and go to sleep in the right phase. Most of the time, sufficiently long wakefulness and hitting the right phase will help you instantly synchronize all sleep variables. However, in some cases, circadian ripples may drag for days, esp. if you are not too fluent in computing your correct sleep phase. If you do lots of shift-work or intercontinental flying, it is very easy to be confused about when your subjective night time occurs. In such cases, you could use SleepChart Freeware to get some visual support that makes a guess easier.
Can sleep impair learning?
Amazingly, the confusion into the types of sleep inertia has been responsible for yet another myth: sleep before learning increases forgetting! Well-timed sleep will not cause sleep inertia and will not contribute to a decline in learning. Just the opposite, it is 20-60 min. after natural waking when the learning results are best. Naturally, this is only true in free running sleep. All too often, alarm clocks are used to interrupt the night sleep and the early morning is pretty unconducive for learning.
Why naps cause sleep inertia?
Naps will cause sleep inertia only if they are taken:
- too late, or
- in conditions of severe sleep deprivation, or
- in conditions of REM sleep deficit.
All those three conditions can fool the sleep control systems into thinking that the nap is the opportune time for launching a full-night sleep episode. If an attempt to launch full-blown sleep takes place long before the main circadian low (nighttime acrophase), you may wake up prematurely with the sense that you got an incomplete and unrefreshing nighttime sleep. Such sleep will leave you groggy and will make it harder to initiate proper sleep during the subjective night. To avoid sleep inertia associated with napping then, avoid sleep deprivation in the first place, and read about the optimum time window for napping.
Long sleep and sleep inertia
Many people believe that long sleep causes sleep inertia, headaches, etc. The root cause of problems that follow long sleep is prior sleep deprivation or sleeping in a wrong phase. Unusually long sleep is simply not possible in a healthy individual on a free running schedule. It is usually a severe sleep deprivation that makes it possible to fall asleep well ahead of the optimum circadian bedtime. The unusually long sleep will then carry through the subjective evening and the entire subjective night, adding up to some highly unusual sleep totals (12-18 hours). Such sleep is often followed by a state that is reminiscent of sleep inertia (the "worn-out" syndrome). No wonder it is easy to build a wrong association between long sleep and sleep inertia. It is very difficult to persist in a long-sleep routine, since the sleep-regulating mechanism will quickly bring the length of sleep to a more typical range. On one hand, the "worn-out" syndrome might seem to persist if the sleep period is wrongly adjusted to the circadian cycle. On the other hand, the "worn-out" observation is usually produced by those who cannot get enough sleep during the week and then sleep long on the weekend. In the latter case, follow-up observation is often impossible due to the next week's obligations. This deepens the wrong conviction that too much sleep is harmful. Healthy individuals cannot get "too much sleep"! Their brain will simply produce natural waking up at the right time. Drs Jim Horne and Daniel Kripke may claim otherwise. Perhaps they never tried to nod off at a peak alertness window?