Sleep glossary
Sleep onset
Where sleep onset actually happens, why the boundary is blurry enough that people misremember it, and how a phone guesses at the moment.
In short
Sleep onset is the moment you cross from wake into sleep. In a lab it is the first epoch scored as sleep after lights out. It is a transition rather than a switch, and most people misremember when it happened.
Sleep onset is also called falling asleep or sleep onset time.
Sleep onset sounds binary and is not. The brain does not flip, it drifts. Thinking turns loose and image-like, muscles let go, sometimes with the hypnic jerk that yanks you back for a second, and breathing settles into something slower and more regular. All of that belongs to N1, the lightest stage of NREM sleep, and it can go on for minutes.
That blur is why people are poor witnesses to their own sleep onset. Wake someone out of early N1 and a good share of them will tell you they were still awake. It is a real experience rather than a lie: the line the scoring rules draw and the line your memory draws are not the same line. Most researchers treat the arrival of N2, marked by the first sleep spindle, as the firmer point where the brain has genuinely stopped tracking the room.
How sleep onset is scored
In a lab, onset is the first epoch scored as sleep after lights out, and the minutes before it are your sleep latency. Almost everything downstream hangs off that one timestamp. Total sleep time, efficiency and wake after sleep onset are all counted from it, so a shift of ten minutes in where onset is placed moves every other number on the report.
A phone has no EEG, so it infers. Movement goes quiet and stays quiet, breathing sound becomes regular, and the app puts onset where those two agree. Tock does exactly this, and it is an estimate: it will tend to place onset slightly early in someone who lies very still while awake, and slightly late in a restless sleeper.
The practical version is that onset is not something you can force, only something you can make likely. Arrive in bed with enough sleep pressure built up across the day, at a time your body clock agrees with, in a room that is not asking anything of you. If yours keeps landing later than you want it to, how to fix your sleep schedule is the fix, and it works over days rather than in one heroic early night.