Sleep, sleep timing and chronotype in animal behaviour

In humans, sleep duration and sleep timing have been identified as interesting facets of individual differences and of personality. Sleep duration and sleep timing are different constructs. For example, compare two individuals both sleeping for 6 h: one may sleep from 2300 to 0500 hours and the other from 0100 to 0700 hours. One can assess the midpoint of sleep in these two subjects which is the midpoint in clock time between sleep onset and awakening. These ideas have already been picked up in a handful of studies. Sex differences have been found in birds in the same direction as in humans with males sleeping for less time. Contrasting effects have been found in relation to mating: in humans, late chronotype men have the highest mating success, whereas in birds, earlier chronotypes gain higher mating success. Many sleep parameters are related to assortative mating in humans and similar but weaker relationships have been found in birds. Ontogenetic studies show that sleep–wake behaviour changes during adolescence in humans. Such changes have also been found in other mammals ranging from primates to rodents. Factors determining sleep–wake patterns could be environmental, such as temperature, sunrise or sunset or even artificial light at night. Artificial lighting at night leads to an earlier wake up and singing time in blackbirds, Turdus merula, whereas light at night makes humans wake up later. As a conclusion, I show some parallels and differences in the study of sleep timing between animals and humans and encourage further studies, in both field and laboratory settings.