Sleep, Circadian Rhythms, and Interval Timing: Evolutionary Strategies to Time Information

A crucial property of the brain is to integrate temporal information with accurate physiological responses (Hinton and Meck, 1997; Buhusi and Meck, 2005; Coull et al., 2011). Evolution has favored biological clocks that dictate homeostatic processes (e.g., the circadian timing of sleep) and, on a smaller time-scale, timed behavioral responses (e.g., interval timing). The interplay between such time-keeping mechanisms is intriguing but biologically complex. Moreover, in biology, analogous problems can be successfully solved by multiple computations. In this article I will discuss of sleep, circadian rhythms, and interval timing by delineating several aspects that suggest a common evolutionary role in providing neurobiological mechanisms for temporal information processing. Neither interval timing nor the homeostatic regulation of sleep are currently as well-understood, at the molecular level, as the circadian clock. This has triggered a scientific interest in linking these phenomena to the circadian molecular machinery. On one hand, sleep homeostasis has been investigated in genetic and lesion studies of the circadian “master” clock (Franken and Dijk, 2009) to test whether the two processes (homeostatic and circadian) were independent. On the other hand, it has been questioned whether interval timing and circadian clock share similar oscillatory mechanisms, although with a different time-scale (Crystal, 2001, 2006a,b; Crystal and Baramidze, 2007), and whether circadian rhythms affect interval timing. Whilst the research in sleep and circadian clock over the last few years resulted in some interesting positive associations (reviewed in Tucci and Nolan, 2010), the question whether interval timing is related to the circadian clock has not a clear unanimous conclusion yet.