Sleep deprivation among adolescents is epidemic. We argue that this sleep deprivation is due in part to pubertal changes in the homeostatic and circadian regulation of sleep. These changes promote a delayed sleep phase that is exacerbated by evening light exposure and incompatible with aspects of modern society, notably early school start times. In this review of human and animal literature, we demonstrate that delayed sleep phase during puberty is likely a common phenomenon in mammals, not specific to human adolescents, and we provide insight into the mechanisms underlying this phenomenon.

A major factor contributing to the evolution of mammals was their ability to be active during the night, a niche previously underused by terrestrial vertebrates. Diurnality subsequently reemerged multiple times in a variety of independent lineages. This paper reviews some recent data on circadian mechanisms in diurnal mammals and considers general themes that appear to be emerging from this work. Careful examination of behavioral studies suggests that although subtle differences may exist, the fundamental functions of the circadian system are the same, as seems to be the case with respect to the molecular mechanisms of the clock. This suggests that responses to signals originating in the clock must be different, either within the SCN or at its targets or downstream from them. Some features of the SCN vary from species to species, but none of these has been clearly associated with diurnality. The region immediately dorsal to the SCN, which receives substantial input from it, exhibits dramatically different rhythms in nocturnal lab rats and diurnal grass rats. This raises the possibility that it functions as a relay that transforms the signal emitted by the SCN and transmits different patterns to downstream targets in nocturnal and diurnal animals. Other direct targets of the SCN include neurons containing orexin and those containing gonadotropin-releasing hormone, and both of these populations of cells exhibit patterns of rhythmicity that are inverted in at least one diurnal compared to one nocturnal species. The patterns that emerge from the data on diurnality are discussed in terms of the implications they have for the evolution and neural substrates of a day-active way of life.

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.