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This is actually a popular question along google results but the answers usually sound like:

  • sleep is voluntary (oh is it?) and healthy
  • fainting is result of accident and is not normal

I read those answers and I'm like: "You don't say?". So yeah, I know that falling a sleep is a result of some preparation to planned unconscious state, whereas fainting is some defensive reaction trying to protect you from moving (or maybe thinking) when you shouldn't.

But I fainted once and when I woke up it was pretty much like waking up from sleep - including the fact I don't remember the moment I fainted. This gave me an idea that maybe, if you faint in your bed, you'll just fall asleep and wake up in the morning, which would be actually pretty sweet.

Therefore my question is, what's specifically going on when you faint and when you fall asleep. Is there any part when these two situations merge and the final state is the same - you're asleep?

And to my original idea: If you hold your breath to the point where you faint in your bed, what will the result be?

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  • $\begingroup$ I'm afraid this wont be easy to answer, because, as far is I know, the process of falling asleep is not fully understood by now. But there must be a difference between those two states, because you can actually become conscious while sleeping (this is referred to a lucid dreaming state) And you can even stay conscious while falling asleep, when using a methode called "wake induced lucid dreaming (WILD)". However, fainting is defined as a unconscious state. $\endgroup$
    – AstronAUT
    Nov 4, 2015 at 13:56
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    $\begingroup$ Perhaps read the Wiki article on sleep and on your kind of fainting. Falling asleep is typically less fast, in the order of one or a few minutes. Sleep is based on the circadian rhythm and body warmth, so people that faint wake up rather fast, as the brain is not primed to rest at that time. There are three broad categories of causes: heart or blood vessel related, reflex also known as neurally mediated, and orthostatic hypotension. $\endgroup$
    – bandybabboon
    Jan 29, 2018 at 9:51

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For this answer I assume that we are talking about sleep in healthy individuals as well as fainting as an isolated event without underlying disease or injury.

The most common form of what we call "fainting" is vasovagal syncope. It is caused by a drop in blood flow to the brain (cerebral hypoperfusion) e.g. by (false) activation of the parasympathetic nervous system and inhibition of the sympathetic system, resulting in low blood pressure or drop in heart rate (reviews 1,2,3). This can be triggered by the Bezold-Janisch reflex. Syncope can also be a symptom of underlying diseases or a side effect of drugs.

The reduced blood flow results in a transient loss of consciousness, a 'shut down' of the brain, which we don't really understand. It is sometimes accompanied by a flattening of an EEG signal, even though the data doesn't seem to be very consistent. Consciousness is defined by two factors, arousal and awareness. The fainted person cannot be 'woken up' and is not aware of the situation (as you said, you don't remember the moment of fainting). But this state normally only lasts for a few seconds or 1-2 minutes.

Sleep, on the other hand, is a mechanism highly regulated by the brain, circadian rhythms and hormones. The brain is thought to switch between the states of sleep and wakefulness by an interplay of different sleep-promoting and wake-promoting areas, including the hypothalamus.

You can argue that sleep is not a state of unconsciousness, since dreaming or even lucid dreaming are connected to a conscious experience. While sleeping lasts longer than the unconsciousness during syncope, it can be reversed by an external stimulus. The sleeping person can be woken up, even though the arousal threshold is increased (loud noises from the alarm are needed). The brain, in fact, is highly active during sleeping (as can be seen in characteristic EEG signals), with energy use close to the state of wakefulness in REM sleep and about 85% during non-REM sleep.

Since these are two different mechanisms, you should not be able to switch between them. After fainting you should regain consciousness very fast. Activation of sleep mechanisms is unlikely in a healthy individual, where sleep regulation is normal and syncope is a rare event.

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  • $\begingroup$ Most of this is good. I would note, that you don't have to restrict this to vasovagal syncope. The final common pathway of syncope is reduced blood flow to the brain, and the EEG shows a characteristic series of changes (if you manage to record during the loss of consciousness). $\endgroup$
    – De Novo
    Mar 7, 2019 at 19:04
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    $\begingroup$ Two other suggestions for improvement. There is no need to preface this with a "we don't understand the brain" statement. We understand it well enough to answer this question unequivocally :) $\endgroup$
    – De Novo
    Mar 7, 2019 at 19:05
  • $\begingroup$ Finally, your last paragraph is a little speculative, though probably partially true. I would consider cutting it. The key (which you've described) is that sleep and fainting are very different brain states, with characteristic patterns on EEG, and behavioral differences (e.g., you can wake someone from sleep). I don't think you need to add a statement about whether it is ever possible to transition from syncope to sleep or sleep to syncope . This is different from the side question of "is fainting a good way to fall asleep?" $\endgroup$
    – De Novo
    Mar 7, 2019 at 19:13
  • $\begingroup$ This is a nice answer. Fainting is involuntary loss of consciousness. During sleep, there is very little difference in the brain between awake and asleep states in that the brain is quite active. $\endgroup$
    – anongoodnurse
    Mar 8, 2019 at 3:11
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    $\begingroup$ @DeNovo I have edited some parts for more information or less speculation. I added the last paragraph since it was part of the question to use fainting as a way to fall asleep. I think that any reported connection between sleep and syncope does refer to 'healthy individuals', which I tried to focus on here. There are a lot of reports about e.g. epilepsy, syncope and sleep, but this is beyond the scope of the question. $\endgroup$
    – Frieke
    Mar 8, 2019 at 19:38

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