Is a sleep cycle always 90 minutes?

No, a sleep cycle is not always exactly 90 minutes. The 90-minute figure is a convenient average commonly used by sleep calculators, but actual cycles often last between about 80 and 100 minutes and may fall outside that range. Cycle duration can vary from person to person and from one cycle to the next during the same night. Earlier cycles tend to contain more deep sleep, while later cycles usually contain more REM sleep. Age, stress, sleep loss, alcohol, medications and health conditions can also affect sleep architecture. Planning sleep entirely around fixed 90-minute blocks may therefore be misleading. A consistent schedule and enough total sleep are generally more important than waking at a mathematically calculated cycle boundary.