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When we sleep there are many different influences that affect the quality and type of sleep that we get. One of the things that our body does naturally is to go through rhythms that regulate the sleep we get and when we need to go to bed. The two main rhythms are called circadian and ultradian.
Circadian rhythms
The term ‘circadian’ refers to a period of time of approximately 24 hours. Therefore, a circadian rhythm is one which occurs daily such as the sleep-wake cycle, fluctuations in body temperature and some hormonal changes. This type of rhythm is controlled by an area of the brain known as the suprachiasmatic nuclei and responds to external influences like sunlight.
- Body temperature: a person’s body temperature should be naturally synchronised with their sleep-wake cycle. Someone’s body temperature will drop when they are ready to sleep and will rise when they are ready to wake up. Body temperature often drops in the middle of the day too, which is a possible explanation as to why some people experience a ‘slump’ at this time
- Sleep-wake cycle: this cycle lasts approximately 24 hours and starts from being awake to sleeping – a pattern which is usually fairly consistent. Two things are known to affect the sleep-wake cycle: jet lag and shift work. Jet lag happens when we cross time zones and because our body is tuned to react to cues, such as darkness. We may not be ‘set’ to sleep when it goes dark because in our heads we are still in the previous time zone where it would be daylight. The same applies to shift work – when it goes dark, people become naturally ready for sleep but some shift workers will just be starting work at this time; this can have a serious impact on their biological rhythms.
Ultradian rhythms
Ultradian rhythms are much shorter than circadian rhythms; they are rhythms which are repeated several times a day, such as the sleep-wake cycle whose stages last approximately 90 minutes.
Other examples of ultradian rhythms are heart rate and eating patterns, which may cause someone to wake up because they are hungry.
