Reading about circadian rhymth in flies I happened to see the Neuron paper by Gandhi et al. about melatonins role in fish. Melatonin – the hormone of the pineal gland – has been shown already to be active in the determination of seasons, its amount produced during the night being measure in so-called calendar cells in the vicinity of the hyphyseal stalk. Now the authors in Pasadena show that melatonin is necessary to fall asleep: zebrafish without the critical enzyme of melatonin systhesis: aanat2 (arylalkylamine N-acetyltransferase 2) take much longer to fall asleep and do not sleep as long as control animals.
Whether the data do apply to men and mammals is open. This is, however, a nice piece of work. It does not explain while I can start sleeping extensively during day time when there is not any melatonin in my circulation.