Adults have an intrinsic body clock which regulates a complex series o
f rhythms including sleep and wakefulness, fatigue and cognitive abili
ty. This endogenous clock naturally runs more slowly than the solar da
y and is entrained to a 24-h rhythm primarily by the alternation of Li
ght and darkness. Jet lag, shift-work sleep disorder, and some of the
chronic insomnias are caused by a temporal discrepancy of the body clo
ck relative to the surrounding environment and social network. The und
erlying mechanisms and general management are described. Both bright l
ight and melatonin therapy have potential in the management of these d
isorders. Traditionally, bright Light therapy has been used to allevia
te the depression associated with seasonal affective disorder. Melaton
in has received much ill-formed publicity, it being claimed that it is
a panacea and an 'anti-ageing' treatment. Both of these treatment app
roaches are reviewed.