Sleep Cycle Calculator: What Time to Go to Bed and Wake Up

What are sleep cycles and stages of sleep?

Our brains undergo a series of sleep cycles while we sleep. The average person requires 5 to 6 cycles to wake up fully refreshed in the morning. Each sleep cycle spans approximately 90 minutes and encompasses five distinct stages. The initial four stages are non-REM stages that comprise light sleep stages in stages 1 and 2 and deep sleep stages in stages 3 and 4. These are followed by the final REM (rapid eye movement) sleep stage (1).

The duration of each stage fluctuates between 5 and 15 minutes. Deep sleep stages tend to be lengthier than REM sleep during the early part of the night, though this pattern reverses as the night advances. Typically, the brain transitions through stages of light sleep, deep sleep, REM, and then back to stages of light sleep and deep sleep, rather than moving linearly from stage 1 to stage 5 (1).

Light sleep stages (N1, N2) are characterized by muscle twitches and increased susceptibility to awakening. These stages mark the gradual preparation for deep sleep and are accompanied by a deceleration of brain wave activity (1).

Deep sleep stages (N3, previously subdivided into N3 and N4) are also referred to as delta sleep or slow wave sleep. Awakening someone during this stage is particularly challenging. Deep sleep is the most crucial phase for rejuvenation and significantly reduces the need for additional sleep. This is why prolonged daytime naps, which can lead to deep sleep, often result in reduced nighttime drowsiness. Additionally, growth hormones play a vital role in restoring the body and muscles during this phase (1).

The REM sleep stage (R) is where dreams occur. The brain exhibits wave patterns resembling wakefulness, with rapid eye movements while the eyes remain closed (1).