Rhythms and EZ/ED

• Circadian rhythm


  Biological rhythm lasting around 24 hours, regulates sleep-wake cycle, hormone release, body temperature
• Endogenous pacemakers (EP)


  Internal body clocks that keep biological processes to time
• Exogenous zeitgebers (EZ)


  External cues that entrain internal body clocks
• Suprachiasmatic nucleus (SCN)


  Master clock for sleep-wake cycle, part of hypothalamus
• Light is the primary EZ for the sleep-wake cycle, but there are others like social cues
• Experiments in caves show the circadian rhythm is free-running but needs entrainment to 24-hour cycle
• Artificial light can disrupt the circadian rhythm by acting as an EZ
• Transplanting the SCN between hamsters shifted their sleep-wake cycle, suggesting it is the EP
• Infradian rhythm


  Biological rhythm longer than 24 hours, e.g. menstrual cycle
• Menstrual cycle


  28-day cycle regulated by hormones estrogen and progesterone
• Pheromones may act as EZs, synchronizing menstrual cycles in women living together
• Cohabiting lesbian couples did not synchronize menstrual cycles, contradicting some studies
• Ultradian rhythm


  Biological rhythm shorter than 24 hours, e.g. stages of sleep
• Stages of sleep


  N1, N2, N3 (non-REM) and REM, cycle repeats 4-5 times per night
• Slow wave sleep is for physical recovery, REM sleep is for mental recovery
• Circadian and ultradian rhythms may be connected, using the same internal body clock (SCN)
• Technology based on understanding sleep stages can help improve sleep quality and productivity