ultradian rhythms

• Ultradian rhythms are biological cycles lasting less than 24 hours and one example is the cycle of brain activity during sleep (sleep stages).
• Sleep has 5 stages occurring through the night, lasting for about an hour in infancy to 90 minutes by adolescence. Sleep is a different state of consciousness, where responsiveness to the external environment is reduced. It occurs daily as a circadian rhythm and is composed of an ultradian cycle of separate stages within the sleep period itself.
• With the invention of the Electroencephalograph (EEG), psychologists could investigate brain activity occurring during sleep, concluding that it was composed of identifiably different sequenced stages with the first four known as NREM sleep while the fifth stage is known as REM sleep
• • Stage one: Alpha waves disappear and are replaced by low-voltage slow waves. Heart rate declines and muscles relax. This is light sleep and people are easily woken.
• • Stage two: A deeper state, from which people are still easily woken. Bodily functions slow down and blood pressure, metabolism and cardiac activity decrease
• • Stage three: Sleep becomes increasingly deeper and people are difficult to wake. Sleep spindles decline, being replaced by long, slow delta waves. Heart rate, blood pressure and temperature decline.
• • Stage four: Deep sleep, where delta waves increase and metabolic rate is low. People are difficult to wake. Growth hormones are released and this is where most of the “repair work” is done. Incidences of sleep- walking and night terrors may occur. Stages 3 and 4 are called Slow Wave sleep. The sleeper spends about 40 minutes in stage four sleep, about an hour passing in total from stage one to stage four. Stage three is re-entered, then stage two and then an active stage of sleep called rapid eye movement (REM), about 90 minutes after falling asleep.
• • Stage five: Also known as REM sleep. After 15 minutes of REM sleep, the sleeper re-enters stages two, three and four in that order, then another cycle begins. It is common to go through about five Ultradian cycles in one night. As the night progresses, the sleeper spends more time in REM sleep and less time in other stages. This pattern is fairly universal, though there are developmental differences.
• Kleitman referred to the 90 minute sleep cycle as the Basic Rest Activity Cycle or BRAC.

  Kleitman suggested this 90 minute cycle also occurred during the day when were were awake however instead of the sleep stages, people progressed from a state of alertness to physiological fatigue every 90 minutes. Kleitman suggested that mind could only concentrate for up to 90 minutes and as this is reached the body runs out of resources which results in fatigue, lack of concentration and hunger.