Sleep glossary
Microsleep
Brief involuntary lapses into sleep that last a few seconds, why you usually cannot tell they happened, and why they are the most dangerous symptom of sleep debt.
In short
A microsleep is a brief involuntary lapse into sleep, usually a few seconds long, that happens when sleep pressure is high. You often have no memory of it, which is exactly what makes it dangerous behind a wheel.
Microsleep is also called micro-sleep or sleep attack.
A microsleep is a short, involuntary intrusion of sleep into wakefulness. It arrives when sleep pressure is high enough to overwhelm the systems holding you awake, it usually lasts a few seconds, and the defining feature is not the duration but the amnesia: most people have no sense that anything happened. You do not experience a gap. You experience continuity that was not there.
That combination is what makes it the most consequential symptom of sleep debt. Every other sign of being short on sleep is something you can notice and compensate for. This one removes the noticing. A few seconds of not receiving information is unremarkable at a desk and is a considerable distance at speed, which is why drowsy driving is treated seriously by transport safety bodies and why “I felt fine” is not evidence of anything.
Why you cannot self-assess it
The awkward finding underneath all of this is that self-rated alertness stops tracking measured performance under chronic short sleep. People adapt to how being sleep deprived feels long before they adapt to being sleep deprived, and the sensation of having got used to six hours is largely the sensation of having stopped noticing.
There is no app-based test for this and Tock does not offer one. What a sleep tracker can honestly do is remove the guesswork from the input rather than the output: how much you actually slept, as opposed to how long you were in bed and how the night felt in the morning. Sleep tracking without a wearable covers what a phone can and cannot establish about that.
If you are experiencing lapses like this during the day on top of what you believe is adequate sleep, that is a medical question rather than a scheduling one. Tock estimates sleep and is not a medical device, and nothing here diagnoses anything.
Questions people ask
How long does a microsleep last?
Typically a few seconds, sometimes up to around thirty. The length matters less than the fact that you are not receiving information for the duration, and that you frequently do not register that it happened at all.
Can you have a microsleep with your eyes open?
Yes. Eyes open and staring is one of the ways it presents, which is part of why it is so hard to self-detect. The absence of a felt gap is not evidence that no gap occurred.