Ultradian Rhythms



  • Lasts less than 24 hours, e.g. brain activity during sleep. Whilst the sleep/wake cycle is a circadian rhythm, the individual sleep stages are ultradian rhythms.
  • Stage 1: light sleep, heart rate declines, muscles relax. Theta waves appear. Approx 15 mins
  • Stage 2: deeper sleep, noiceable bursts of sleep spindles. Theta waves get slower and larger, bursts of k-complexes (activity). Approx 20 mins
  • Stage 3: increasingly deep, sleep spindles decline, quite difficult to be woken from. Long slow delta waves. Approx 15 mins
  • Stage 4: deep sleep, low metabolic rate, growth hormones released, very difficult to be woken from. Sleep walking may occur
  • REM: noticeable eye movements and dreaming occurs, brain oxygen and glucose demands increase, most difficult to be woken from, go back through stages 2-4 - this cycle repeats throughout the night.
  • Nature vs Nurture - it can be assumed that ultradian rhythms of sleep are a part of our nature as everyone has them. Animal studies have found that when lesions are made in areas of the brain that controls circadian rhythms, ultradian rhythms are not effected, implying that they are governed by differently control mechanisms. Ethical issues arise in animal research as the research can involve permanent damage/death which is extremely unethical. The findings are also not necessarily generalizable past animals, meaning they cannot be extrapolated to humans and therefore lack external validity.



  • STUDY: Dement and Kleitman
  • Measured electrical activity in the brain during sleep using EEG machines and woke pp's up during different stages of sleep. 90% of people awakened during REM reported dreaming, only 7% NREM.
  • Objective and controlled some variables e.g. no caffeine
  • Confounding variables - individual differences due to length of pp's usual sleep cycle
  • Low ecological validity - sleep lab, artificial conditions, doesn't reflect home environment
  • The link between REM sleep and dreaming is important for researchers to investigate dreaming itself. However, this is arguably deterministic in assuming that REM always results in dreaming, which is not always the case. The theory is therefore also reductionist as it doesn't consider more complex contributing factors.


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