Sleep isn’t uniform. Once we fall asleep, our bodies follow a sleep cycle, lasting 90-120 minutes on average. Our total sleep is made up of several rounds of this cycle. Each cycle contains four stages. The first three stages are known as non-rapid eye movement (NREM) sleep, and the final stage is known as rapid eye movement (REM) sleep.
Stage 1 NREM
Recognised as the “dozing off” stage, it lasts around one to five minutes. Muscles relax and your heart rate, breathing, and eye movements begin to slow down, as do your brain waves, which are more active when you are awake. It’s easy to wake someone up during this stage, but undisturbed, they can move quickly into stage 2.
Stage 2 NREM
It’s characterized by deeper sleep as your heart rate and breathing rates continue slowing down. Eye movements cease and body temperature decreases. Apart from some brief moments of higher frequency electrical activity, brain waves also remain slow. Stage 2 is typically the longest of the four sleep stages.
Stage 3 NREM
This stage plays an important role in making you feel refreshed and alert the next day. Heartbeat, breathing, and brain wave activity all reach their lowest levels, and the muscles are as relaxed as they will be. This stage lasts longer in initial cycles, decrease in duration over the following cycles.
REM
It occurs ~90 minutes after falling asleep. Your eyes move back and forth rather quickly under your eyelids. Breathing rate, heart rate, and blood pressure begins to increase. Dreaming typically occurs during REM sleep, and your arms and legs become paralyzed – it’s believed this is intends to prevent us from physically acting out our dreams. The duration of each REM stage increases by cycle, the inverse of stage 3 NREM. Studies linked REM sleep to memory consolidation, the process of converting recent experiences into long-term memories. The duration of the REM stage decrease as we age, causing us to spend more time in the NREM stages.
Moments where we briefly wake during the night, but not remember them, are known as “W” stages.