the sleep cycle(bio rythms)

• Endogenous pacemaker


  Something inside that has the ability to keep time, an internal body clock
• Exogenous zeitgeber


  An environmental cue from the outside world that keeps our internal body clock on time, also called entrainment
• Circadian rhythm


  A biological rhythm that lasts around 24 hours
• The sleep/wake cycle is the most obvious circadian rhythm, but we also have circadian rhythms for releasing hormones and varying body temperature
• Sleep-wake cycle


  • We have a period of sleep and a period of wakefulness every 24 hours
• Exogenous zeitgebers (cues like light, clocks, social cues)

  Can conflict with the internal body clock, leading to issues like jetlag
• Suprachiasmatic nucleus (SCN)


  The endogenous pacemaker for the sleep/wake cycle, also known as the master clock
• Light detected by the SCN

  Suppresses melatonin production in the pineal gland, making you feel less sleepy
• The main exogenous zeitgeber for the sleep/wake cycle is the amount of light received in the eyes
• Social cues like meal times and environmental cues like clocks also work as exogenous zeitgebers
• Free-running circadian rhythm


  The endogenous biological rhythm maintains itself without entrainment by external cues
• Studies on free-running circadian rhythm

  • Siffre's 179-day stay in a cave without access to natural light or clocks
  • More recent studies controlling for artificial light use
• Evidence suggests the SCN is the biological structure responsible for the circadian rhythm in the sleep/wake cycle
• Disruption to sleep patterns can lead to increased anxiety, decreased alertness, and decreased vigilance
• Understanding how the sleep-wake cycle is affected by exogenous zeitgebers can help develop treatments for jetlag, improve performance of athletes who travel, and help shift workers be more productive
• Knowing the circadian rhythms like blood pressure can help with timing drug treatments, benefiting health services
• Exposure to blue light at night and from personal devices has implications for maintaining healthy sleep
• Even with social cues, a man blind from birth had a 24.9 hour circadian rhythm, out of sync with the rest of society
• There is an evolutionary explanation for the day/night cycle of humans and other animals specialized for daytime activity
• Nocturnal animals like rats have a circadian pacemaker shifted 12 hours, so their wakefulness is at night