The French Medicine and Research Sleep Society had organized a consensus conference about sleep/wake circadian rhythms and their disorders. During this conference a subgroup of 11 sleep doctors/researchers looked specifically at the use of MEL in different pathologies. This article gives a summary of the main results of MEL therapy in some neurological diseases and insomnia approved by this consensus group. Exogenous MEL, which crosses the blood-brain barrier, has been used as a treatment in its two available forms: an immediate release form that principally shows a chronobiotic action and a long release form that mimics the physiological MEL secretion rhythm and is used to replace reduced physiological secretion. MEL secretion decreases frequently with age, mostly in elderly insomniacs and dementia patients. Results of level A studies show that MEL therapy, used as an add-on treatment, has beneficial effects in mild cognitive impairment (MCI) and Alzheimer patients with sleep disorders in improving sleep quality and in regulating the sleep/wake rhythm. MEL has to be prescribed as early as possible and for a long period, at a dose of 2 to 5 or 10 mg. It may have a beneficial effect on cognitive function in MCI but shows no effect in moderate to severe Alzheimer's disease. It should be emphasized that there are no serious side effects with MEL treatment. In these diseases, light therapy used 12 hours before melatonin treatment has a positive synergic effect. In REM sleep behavior disorder, immediate release MEL should be prescribed first as its side effect profile is much better than clonazepam shortly before bedtime. MEL has a good efficacy on clinical symptoms and PSG REM sleep without atonia episodes and is well tolerated. In Parkinson disease with sleep disorders and without REM sleep behavior disorder, MEL seems to improve subjective sleep quality but no conclusions can be drawn. There is insufficient scientific proof for using MEL as a prophylactic treatment in primary headache, migraine and cluster headache. In epileptic patients, MEL can be safely used to regulate the sleep/wake rhythm and to improve insomnia but more randomized controlled studies are necessary. In primary or no-comorbid insomnia, only a 2 mg dose of slow release MEL, 1 to 2 hours before bedtime, over a period of 3 to 12 weeks, is recommended. It decreases sleep onset latency, improves quality of sleep, morning alertness and quality of life without serious side effects and without withdrawal symptoms.