Circadin (Melatonin) is a prolonged release melatonin supplement which can be taken to assist with insomnia and, after the positive effects on jet lag were studied in the Cochrane Review, it can also be used by anyone suffering from sleep issues after changes in time zones due to long flights.
The hormone melatonin is released naturally from the pineal gland during the evening and through the night to help you get to sleep, and stay asleep whilst it is dark. During the day when it is light, melatonin production is suppressed, and this is how the body regulates its sleep/wake cycle.
After crossing into different time zones, the time you would normally start thinking about going to bed, around 8 pm or 9 pm, may now be the middle of the afternoon or night in the country you have travelled to. As such, your body clock becomes confused as at first, melatonin still naturally begins to be produced around the same time as normal. This is your normal circadian rhythm which is a twenty four hour cycle during which physiological processes naturally occur such as going to sleep and waking up. You can wait until your jet lag passes naturally and your body gets back in line with its new time zone, but this can take several days. As such, some people find enormous benefit in using a melatonin supplement to help top up the body when it is not producing this hormone at the right time, or enough of it, to induce sleep.