What happens in your brain each night while you sleep.
Find your ideal time to sleep or wake up
Calculate my sleep cycles →Each sleep cycle lasts approximately 90 minutes and is made up of 4 stages:
This is the transition between being awake and asleep. Muscles relax, breathing slows down, and you may experience muscle spasms (hypnic jerks). It is easy to wake up during this stage.
Body temperature drops and heart rate slows. The brain produces "sleep spindles" — bursts of activity that help consolidate memory. You spend approximately 50% of the night in this stage.
The most restorative stage. The body repairs tissue, strengthens the immune system, and releases growth hormone. It is very hard to wake up during this stage, and if you do, you will feel strong sleep inertia (grogginess).
REM stands for "Rapid Eye Movement." This is where the most vivid dreams occur. The brain is almost as active as when awake, but the muscles are paralyzed (so you do not act out your dreams). This stage is crucial for emotional memory and creativity.
The ideal time to wake up is at the end of the REM stage or during stage 1–2 (light sleep) of the next cycle. Our calculator computes exactly those moments.