Understanding sleep
Sleep cycles: not an exact 90-minute timetable
By Sömnlyftet · created with AI support Updated:
The main answer
During the night, you move through different sleep stages in recurring cycles. The cycles are not all exactly the same length. Counting backwards in fixed 90-minute blocks cannot tell you with certainty when it will be best to wake.
A cycle contains different stages
Sleep is commonly divided into REM sleep and three stages of non-REM sleep. Deep sleep is the third non-REM stage. The stages differ in patterns of brain activity, among other features. A night is therefore not one uniform state of rest.
The balance changes through the night
Deep sleep is more common earlier in the night, while REM periods tend to become longer later. Brief awakenings can occur between cycles. Remembering an awakening does not, by itself, describe how the entire night went.
Prioritise enough sleep over a perfect calculation
NHLBI describes cycles of roughly 80–100 minutes, but this is not a personal timetable. Sleep onset and variation during the night make exact calculations uncertain. Do not shorten a night you need simply to make the number of minutes fit a calculator.
An illustrative example
A calculator suggests six hours to fit four 90-minute cycles. You do not use this as a reason to give up needed sleep. Instead, you plan around when you must get up and how much sleep you generally need.
A common question
Is an uneven cycle chart on my watch a problem?
Not automatically. Cycles vary, and a watch estimates their boundaries. Consider several nights and how you function during the day rather than whether the bars on a chart look perfectly regular.
Sources and authorship
The sources describe sleep and habits in general. They have not evaluated Sömnlyftet. The examples are fictional and illustrate an approach, not the results you will achieve.