Human sleep has been separated into two main types: Non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep
NREM sleep is further divided into four stages, with each stage increasing in their depth of sleep
REM sleep is named this way because of the bizarre, horizontal, shuttling eye movements that occur underneath your eyelids during this stage of sleep
REM sleep is the principal stage in which we dream, although it can occur in the other stages as well
NREM and REM sleep play out in a battle for brain domination throughout the night
This cerebral war is going to be won and lost every 90 minutes, creating the standard cycling architecture of sleep for humans
Other species may have a slightly different NREM to REM cycles, with birds experiencing a cycle that only lasts about 4-5 minutes, and giraffes about 24 minutes
During NREM sleep, we see bursts of deeply powerful brain waves that flow over your cortex
We also see sleep spindles, which are short, synchronous bursts of electrical brain wave activity that last for about 1-1.5 seconds
About 60-70 minutes into your first sleep cycle, your brain starts to rise up again into later stages of NREM sleep, then pop into a short REM sleep period, back to NREM sleep, then returning to REM sleep once again
Your brain goes through this cycle every 90 minutes
What changes is the ratio of NREM to REM sleep as you move through the night, with your sleep cycle comprised of more REM sleep during the second half of the night
If you wake up two hours earlier than usual, you aren’t just losing 2 hours of sleep - you could be losing 50, 60, or even 70% of your REM sleep
All stages of sleep are critical and perform different functions for the brain and body at different times of night
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