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physiology of sleep


  • During a single 24 hour day we have a period of 6-10 hours when we are very sleepy. This is the time when we normally sleep. During the remaining 14-18 hours we are usually awake; however, only a portion of that waking time is suitable for intellectual effort. This period of maximum alertness may last as little as 2-6 hours. We should plan our day in such a way that sleep comes at the time of maximum sleepiness, while activities that demand maximum focus or creativity fall into the hours of maximum alertness. It is very difficult and usually very unhealthy to force your body and your body clock to do what you wish. It is far easier to do the opposite: adapt your life to your body clock. 1)
  • sleep patterns change throughout life
  • number of hours sleep required each night varies significantly between individuals, children and adolescents need 9-10hrs, while adults need at least 7.5hrs.
  • as people age, they tend to have shorter overall sleep period & spend more time in the lighter sleep stages, with less REM sleep, often being interrupted by brief periods of wakefulness
    • older workers do not tolerate irregular shifts or disrupted sleep as well as younger workers
  • until recently, it was thought adolescents needed less sleep than pre-pubertals, but this has been challenged and it is now thought adolescents need 9-10 hours sleep per night to ensure the pubertal neural reorganisation occurs normally.
    • unfortunately, the tendency for later sleep times for adolescents due to both circadian and social events (eg. TV, computer, etc) combined with fixed school start times results in chronic sleep debt for many, and risk of behavioural issues and depression, and perhaps schizophrenia.
  • alertness throughout the day is determined by two main opposing biologic factors:
    • circadian rhythm
    • homeostatic sleep drive
  • determinants of alertness and performance include:2)
    • biological time of day based on circadian cycle
    • consecutive waking hours (homeostatic sleep drive)
    • nightly sleep duration (or sleep debt)
    • sleep inertia - waking after a nap > 20-30min duration (when in NREM sleep) results in a groggy feeling

circadian cycle

  • this is a wake drive for alertness which in humans is minimum in early am and maximal in late afternoon (to oppose higher drive for sleep via homeostatic drive, the longer it is that one has been awake)
  • this is a property of all higher life forms
  • humans evolved to work during daylight hours
  • the human circadian cycle is generally 25 hours long, hence it is easier to go to bed later each night than earlier each night.
  • “after-hours” work is a recent societal need that is out of harmony with our evolutionary inheritance
  • this clock can be changed by external factors (Zeitgebers) such as:
    • light (which suppresses melatonin secretion by the pineal gland) which is more potent than social cues
      • bright light (>3000lux) very early in the morning can cause a phase advance, and can reduce fatigue for those forced to work overnight
      • ultrabrief exposures (< 1 msec) to bright light every minute for 60 minutes can also act as a Zeitgeber
    • brief aerobic exercise
  • one can force circadian phase to shift by 1-2 hours each day at most, thus it takes at least a week to adjust to a new shift time!


  • in mammals, a master circadian “clock” resides in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus and slave clocks in most peripheral cell types (eg.kidney, liver, etc).
  • The SCN clock is composed of multiple, single-cell circadian oscillators, which, when synchronized, generate coordinated circadian outputs that regulate overt rhythms.
  • The SCN is entrained by the photoperiod via synaptic connections with the retina (retino–hypothalamic tract, RHT)
  • Our current understanding of the clock architecture consists of 3 negative and 1 positive loop of transcription, translation, and posttranslational events.
  • The daily light-dark cycle ultimately impinges on the control of two clock genes that reset the core clock mechanism in the SCN. Clock-controlled genes are also generated by the central clock mechanism, but their protein products transduce downstream effects. Peripheral oscillators are controlled by the SCN and provide local control of overt rhythm expression.
  • The phase of peripheral clocks can be completely uncoupled from the SCN pacemaker by restricted feeding.
    • Thus, feeding time, while not affecting the phase of the SCN pacemaker, is a dominant Zeitgeber for peripheral circadian oscillators. Glucocorticoid hormones inhibit the uncoupling of peripheral and central circadian oscillators by altered feeding time.3)

molecular basis

  • “Although our understanding of the molecular basis for the circadian rhythm is continually evolving, the current model involves a complex interplay between environmental and endogenous factors, which include a core set of circadian genes. Transcriptional and post-transcriptional interactions among these gene products results in an autoregulatory feedback system, which allows for predictable cycling of the core circadian elements. In addition, many of the circadian genes operate as transcriptional regulators for transcripts outside of the circadian system, and recent evidence indicates that as many as 10% of all mammalian genes may be regulated to some degree by the circadian oscillatory mechanism. As a result, disturbance of the circadian system, either through environmental exposures, or through genetic alterations in the key circadian genes, may have important implications for a variety of biological pathways.”4)


  • Eight clock genes have been cloned that are involved in interacting transcriptional-/translational-feedback loops that compose the molecular clockwork.
  • genetic polymorphisms result in some people being “early birds” while others are “night owls”. Some have a delayed sleep phase syndrome and are “extreme night owls” - these people may respond to agomelatine (Valdoxan), a new antidepressant which also acts on melatonin receptors to cause a phase advance.
  • there are seven well-known variations, or polymorphisms, in human circadian CLOCK genes 5)
  • there appears to be an association with some of these variations with risk of bipolar depression disorders.
  • core circadian CLOCK genes include:
    • cryptochrome 1 (CRY1)
    • cryptochrome 2 (CRY2) - involved in the negative arm of the circadian feedback loop
      • CRY2 knockdown results in increased accumulation of mutagen-induced DNA damage6)
    • period 1 (PER1)
    • period 2 (PER2)
    • period 3 (PER3)
  • in addition there appear to be a multitude of genes involved in peripheral tissues (liver, kidney, etc) which are involved in peripheral slave clocks and which control cell function in relation to diurnal and other rhythms.

homeostatic sleep drive

  • essentially, the drive to sleep increases the longer one has been awake
  • if there has been inadequate sleep overnight (either in duration or quality), daytime sleepiness may be evident even after being awake for only a couple of hours (as circadian wake drive is at a minimum and insufficient to oppose the sleep drive)
  • if the awake period exceeds 18 hours, psychomotor function deteriorates to a level similar to that of a person with blood alcohol level of 0.05-0.10% 7).
  • if the awake period exceeds 24 hours for a doctor, clinical task performance falls to 7th percentile of rested doctors 8), while EEG-documented attentional failures doubled 9) for those doing > 16 hours of clinical duty with a 500% increase in diagnostic errors and 36% more serious errors!10)
  • the relative risk for car accident on commute home for doctors doing an extended shift instead of a normal shift was 2.3! (Barger et al. NEJM 2005; 352:125-134))

clinical ramifications of circadian rhythms

  • there is a well observed temporal incidence in adverse cardiovascular events, including transient myocardial ischemia, myocardial infarction, sudden cardiac death, and stroke,which occur most frequently in the early morning hours just after awakening, but also display a secondary more subtle peak in the late afternoon.
  • these events correlate with diurnal rises in BP, HR, platelet aggregation tendency, fibrinogen levels, LDL and cortisol levels.
  • peak blood pressure levels in humans occur during the mid morning (at about 10:00 AM) then decrease progressively throughout the remainder of the day to reach a trough value the following morning at around 3:00 AM. A slow but steady increase in blood pressure is then observed over the early morning hours before awakening, with an abrupt and steep increase at approximately 6:00 AM coincident with arousal and arising from overnight sleep.11)
William, Dawson
Philibert.I. Sleep 2005; 28: 1392-402
Lockley et al. NEJM 2004; 351:1829-37
Landrigan et al. NEJM 2004;351:1838-1848
Vascular Clocks and Cardiovascular Disease. Arterioscler Thromb Vasc Biol. 2007;27:1694-1705.
sleep_physiology.txt · Last modified: 2011/05/19 17:12 (external edit)