see also: insomnia; sleep hygiene; travel medicine;

This article is a summary of The Lancet Vol 369 March 31, 2007 by Waterhouse et al.

• jet lag is a form of travel fatigue that results from the effects of a new time zone on an unadjusted body clock.
• this results in loss of sleep at night and inappropriate phasing of the daily (circadian) body rhythms.
• it increases with the number of time zones crossed, is worse with older travellers and depends on the direction of the time-zone transition (ie. whether traveling from east to west or vice versa).
• jet lag abates as the body clock adjusts to the new time zone.
• travel fatigue can also be caused by disruption of sleep, the stress of travel and general dehydration, but unlike jet lag, this form of fatigue usually resolves after a good night's sleep.

• the body clock brings about daily rhythms in core temperature, plasma hormone concentrations and the sleep-wake cycle - all of which exert widespread effects on the body and mind.
• the body clock mechanism is located in the suprachiasmic nuclei which are paired groups of brain cells located either side of the midline at the base of the hypothalamus.
• these nuclei have melatonin type 2 receptors and receive information about light from the retinohypothalamic tract.
• there is also input via the intergeniculate leaflet which is believed to carry information about physical activity and general excitement.
• the clock is formed by cyclic interactions between clock genes and clock proteins.
• most of the genetic information of the clock is conserved between species.
• in the absence of external rhythmic inputs & time cues, the clcok and daily rhythms continue but with a period that is not exactly 24hrs. This period is intrinsic to the body clock and appears to be approx. 24.5hrs.
• to be useful to us, this period is adjusted by external cues (zeitgebers) so that it adjusts to the solar day by advancing or delaying the body clock phase.
• the main zeitgebers are:
  • light-dark cycle
    • bright light directly affects the body clock nuclei and indirectly by inhibiting melatonin secretion
    • the inhibition of melatonin secretion appears to have the following properties:
      • suppression is more when light is:
        • in the upper visual field
        • shorter wavelengths (eg. blue-green light)
      • even low intensity light may suppress it if individual recently lived in a dim environment
  • the rhythmic secretion of the pineal gland hormone melatonin which occurs during sleep in healthy people
  • to a lesser effect, exercise
• at about 2100hrs if there is no exposure to bright light, the pineal gland starts producing melatonin (“dim-light melatonin onset” or DLMO) which causes the body's core temperature to start falling and it is this fall in temperature that allows sleep to ensue.
• sleep is easiest at times of falling body temperature or low body temperature, but difficult to initiate and maintain when temperature is rising or high.
• core temperature is normally lowest between 0300hrs-0700hrs.
• waking occurs when core temperature starts to rise.
• mental performance generally improves with rising core temperature but decision-making skills deteriorate with time awake because of fatigue before the evening fall in core temperature.
• many aspects of physical activity appear to display circadian rhythms with performance correlating with core temperature but perhaps the main factor affecting performance is sleep loss and its effect on mood, strategy and desire to train to a maximum.
• thus:
  • exposure to bright light after DLMO (ie. after 2100hrs) acts to delay the onset of sleep
  • exposure to bright light after 0700hrs acts to advance the clock phase
  • taking melatonin prior to midnight advances the clock
  • taking melatonin after midnight delays the body clock
  • exercise after the DLMO will delay the clock
  • exercise between 1700-2000hrs will advance the clock

• plan the journey well in advance
• arrange for any stopover to be comfortable
• arrange documentations, inoculation, etc
• make arrangements at the destination

• take some roughage to eat (eg. apples)
• drink plenty of water or fruit juice rather than tea, coffee or alcohol which will dehydrate you

• relax & rehydrate with non-alcoholic drinks
• take a shower
• take a brief nap if needed, but not enough to stop getting to sleep at night

• minimise travel fatigue as above.
• if less than 3 time zones are crossed, then jet lag is unlikely to be a problem for most people.
• if the stay is too short for adjustment of the body clock (for most people, fewer than 3 days), then remain on home time and attempt to arrange sleep and engagements to coincide with this.
• if the journey is across more than 3 time zones and the stay is for longer than 3 days then:
  • try to book flight times with the least disruption depending on which direction you are flying:
    • see table below
    • body clock adjusts to large delays more easily than large advances hence a 10hr flight to the east should be managed as for a 14h flight to the west.
    • phase advances in excess of 8hrs are difficult to achieve
  • on the plane:
    • adopt the destination time zone when get on the plane
    • avoid sleeping on the plane, unless it is night time at destination
      • although “banking sleep” may seem a good idea, it will tend to further anchor rhythms to your old time zone.
      • to assist with sleep on plane at destination night time, consider taking 3-5mg melatonin 2-3hrs prior to time of sleep.
  • after the flight:
    • maintain daytime alertness:
      • avoid a prolonged sleep during the local day as this will interfere with sleep at night
      • consider use of caffeine if it is early in the day local time (the stimulant modanifil may be useful as it seems to have little adverse effects)
      • gentle exercise in bright light (eg. sightseeing or golf)
    • use bright light to adjust your body clock see table below
    • promote sleep and clock adjustment by taking 3-5mg melatonin 2-3hrs prior to sleep time (but do not use melatonin if young or pregnant)

• the above assumes the person usually has a minimum core temperature at 0400hrs.

• if the minimum core temperature is at 0600hrs then adjust the times by 2hrs, eg. for 3 time zones to the west, avoid light at 0400-1000hr and seek it at 2000-0200hrs.

• MJA 2009: Circadian rhythms: keeping pace with developments