Ultradian rhythms are biological rhythms that repeat more than once every 24 hours.
Sleep stages
Stage 1- falling asleep, can be easilywoken
stage 2- heart rate starts to slow and body temp drops
stage 3+4- fall into deeper sleep, blood pressure and brain activity drop
stage 4- its difficult to wake someone up and wake up confused
REM sleep
Stage five is called rapideyemovement (REM) sleep. During this stage of sleep we are sleeping relatively deeply, however, our brain is very active and our eyes are moving around rapidly. It is during this stage of sleep that we often dream
We cycle through the stages of sleep repeatedly. Each sleep cycle lasts around 90 minutes, so in our usual 7-8 hours sleep a person goes through around 5-6 cycles
As well as the stages of sleep, appetite is an ultradian rhythm as across 24 hours we go through multiple cycles of feeling hungry then full again
Kleitman suggests we go through 90 minute cycles of alertness multiple times in the day where we go from being alert to tired within 90 minutes. This cycle of alertness-sleepiness is called the Basicrestactivitycycle (BRAC). The BRAC is an ultradian rhythm