During depression, chronobiological disorders occur, such as disturbances in body temperature and early urinary excretion of a noradrenaline metabolite. Sleep patterns are disturbed in 90% of depressed patients; early REM sleep and shortened slow-wave sleep (stages 3 and 4), resulting in an increase in REM sleep, have been observed. Thus, an increase in REM sleep may be an indication of depression. Chronic insomnia is characterised by irregular sleep behaviour, an anxious attitude to sleep and increased cognition before sleep onset. Patients with this disorder can be divided into those with a disturbed ultradian rhythm (less than 2 REM-NREM cycles) and those with regular sleep structure (greater than 2 REM-NREM cycles). Most antidepressants reduce REM sleep, an effect evident from day 1 of administration. Trimipramine is an exception in that it has antidepressant and sedative effects without modifying REM sleep, and it possesses a different pharmacodynamic profile. Trimipramine is effective in depressed patients with chronobiological disorders such as chronic insomnia, although its mechanism of action is not fully understood.