Jet lag, also known as 'time zone change syndrome' is a temporary sleep disorder that is caused by the disturbance to the body's circadian rhythm when travelling over time zones.  By entering a new time zone, there is discord between the time that you would naturally want to be asleep and the time in which you would naturally want to be awake.  In essence, you end up trying to undertake activity and rest at times that your internal body clock least expects it. For instance:

Depart London: 11:35am Thursday
Arrive Los Angeles: 00:35am Friday (UK time) but 4:35pm Thursday LA time

So you are ready to sleep straight away, sleeping for the average 8 hours and then waking up at 00:35am Friday - just after everyone else in LA has just gone to sleep! To combat this social inconvenience, most people try to stay awake through their tiredness and sleep again at a reasonable hour.  However, whatever you try - your body clock is now out of sync.

This clock  is in control of the circadian rhythms that you experience, meaning that it tells your body at what time you are ready to go to sleep and then what time to awaken the next day.  By passing through to new time zones so quickly, your body clock does not have the time it needs to adjust and this can lead to:

- sleep disturbances
- loss of normal functionality
- irritability
- loss of alertness
- sickness and a lowered immune system
- menstrual symptoms for frequent travelling women

Jet lag is a temporary condition. Signs of it appear one to two days after air travel across at least two time zones. How severe it is and how long it lasts depends on the number of time zones you cross. It is also related to the direction of travel. Flying east tends to be harder to adjust to than flying west. Estimates are that it takes one day per time zone for your body clock to adjust to the local time. Crossing more than six time zones can require even more time for your body to adapt. Some people are able to adjust more quickly than others to rapid shifts in time zones.

How Can Light Therapy Help?

A large contributor to the symptoms of jet lag is that your body experiences darkness and daylight at the incorrect times during the day.  However, the body can be reset to its new time zone by following a careful program of light therapy which means that rather than taking a week to adjust to a new time zone, it can take as little as one or two days.

The light therapy should be undertaken at different times, depending on the length of the flight and the direction you are travelling in. The following rules have been found to be useful, however, you should always refer to manufacturers guidelines or seek professional advice as improper use can lead to the symptoms becoming worse:

- travelling eastbound across up to six zones: light in the morning;

- travelling westbound across up to six zones: light in the late afternoon;

- travelling eastbound across 6-10 zones: avoid light in the morning and seek light at noon.

- travelling westbound across 6-10 zones: avoid afternoon light and seek it at midday.

Light therapy sessions for approximately 30-60 minutes on the day of travel and for 15-30 minutes on the days after arriving will help a great deal.  It is also possible to plan ahead.  Try to slowly adjust your body clock by shifting the times that you sleep and wake in the days leading up to the trip - the Lumie Bodyclock is excellent for this purpose.