Ultradian rhythms are bodily rhythms that last less than 24 hours. The most obvious ultradian rhythm is the different stages of sleep, existing within the ciradian rhythm of the sleep-wake cycle. Sleep is dived into cycles, each lasting about 90 minutes. During these cycles, we will experience 5 different stages of sleep- the first four being characterized as non-rapid eye movement sleep (NREM), and the last stage being characterized as rapid eye movement sleep (REM).
When human beings are awake, the electrical activity in their brain usually takes the form of fast, erratic brain waves. These waves are known as beta waves. During stage 1 of sleep, the brains electrical activity begins to change, with the brain waves becoming more slow and regular. This demonstrates that the persons mind is beginning to relax, and these waves are known as alpha waves. The presence of alpha waves in the brain can usually be used to indicate that the onset of sleep has occurred, and that the person is no longer awake. As well as a change in electrical activity, the persons core body temperature will also drop and their heart-beat slow down.
As the sleeper goes into Stage 2 of sleep, their brain waves become even slower and a greater amplitude. These are called theta waves. This shows that the brain is becoming increasingly more relaxed. During this stage of sleep, it is common to experience sudden bursts of brain activity: known as K complexes and sleep spindles.
Stages 1 and 2 of sleep are commonly referred to as being as being ‘light sleep’. This means that people in these stages of sleep can be easily awoken by external factors such as noise.
Stages 3 and 4 of sleep are characterized by even slower brain waves, known as delta waves. Stages 3 and 4 are often referred to as being slow-wave sleep (SWS)- and sleep during this time is far more deep than in the previous two stages. However, the sleeper has not reached a state of unconsciousness, and can still be woken up by certain external factors (eg.
Parents being woek nby the sound of their baby crying). These are the stages of sleep where most of the bodies ‘physiological repair work’ occurs. It can also be associated with biochemical changes, such as the release of growth hormone taking place during this time.
In these first four stages of NREM sleep, dreams have been shown to be unlikely to occur. During a study by Dement and Klietman, in which participants were woken up when their brain waves indicated certain stages of sleep, it was shown that only around 7% of participants had reported having dreams when their brain activity had indicated the stages of NREM sleep.
Stage 5 of sleep, the final stage in the sleep cycle before it repeats, is known as REM sleep. This the stage of sleep where brain activity is most like that of the awake brain. It is also the stage of sleep where most dreams are likely to occur.