How sleep cycles work

A typical night moves through several sleep cycles. Each one lasts about 90 minutes and cycles through a few different stages.

The stages of a cycle

1. N1 is the brief drift from awake into sleep. It usually lasts a few minutes.
2. N2 is light sleep. Most of the night is spent here.
3. N3 is deep, restorative sleep. It dominates the first half of the night.
4. REM is when most dreaming happens. REM periods get longer toward morning.

A cycle does not always include every stage in the same order, but the rough pattern repeats four to six times per night.

Why this matters for waking up

Waking out of deep sleep tends to feel groggy. Waking near the end of a cycle, when sleep is lighter, tends to feel easier.

The sleepytime calculator suggests times that line up with the end of a cycle. That is why the suggestions come in roughly 90-minute steps.

What to take away

The exact cycle length varies from person to person and from night to night. The 90-minute figure is a useful average, not a rule. If a suggested time does not feel right, try the next one earlier or later.