Deep sleep

Deep sleep is the third stage of non REM sleep, written N3 in modern scoring and still widely called slow wave sleep after the tall, slow delta waves that define it. In a healthy adult it usually takes up somewhere around a fifth of the night, and the share falls steadily with age, which is part of why older sleepers often describe their nights as lighter.

The more useful fact about N3 is when it happens rather than how much of it there is. Deep sleep is loaded into the front of the night. The first cycle normally carries the longest slow wave stretch, the second carries less, and by the early morning hours N3 has mostly given way to REM sleep. A night cut short at the beginning therefore costs you something quite different from a night cut short at the end.

Why waking out of deep sleep feels so bad

The general finding in sleep research is that sleep inertia is at its worst when you are woken out of slow wave sleep. The grogginess is deeper, the confusion lasts longer, and reaction time takes more time to come back. Because N3 is concentrated early, an alarm at 2am or 3am is far more likely to catch you in it than an alarm at 7am, and the difference is not subtle.

That is the reasoning behind smart wake in Tock. It watches the 30 minutes before your set alarm for a lighter moment and rings then, with your set time always kept as the fallback so you cannot oversleep. It is a small nudge rather than a cure for a short night, but it moves the moment of waking away from the worst possible point.

Beyond the stage itself, sleep architecture is the word for how N3 and everything else arrange themselves across a night, and sleep cycles explained walks through that order in plain language. One caveat on any app that shows you a deep sleep figure, including Tock: on a phone only night those stages are estimates drawn from movement and sound. Measured stages need Apple Health or a wearable.