Why some animals are nocturnal decoded

Scientists have identified differences in brain pathways that explain why some animals sleep at night and are active during the day, while others do the reverse, an advance that may lead to treatments for certain sleep or mood disorders.

Researchers examined the day/night patterns of monkeys (diurnal) and mice (nocturnal). They found that although both process light through the eyes in a similar way, the signals that determine sleep/awake modes are sent to the brain via different routes and produce completely opposite sleep/awake patterns.

"Since humans are diurnal, this has clear implications for potential novel treatment of certain sleep or mood disorders," said Qun-Yong Zhou, from the University of California, Irvine in the US. Sleep/awake patterns are among the basic physiological functions in virtually all organisms that are governed by circadian rhythms.

These time-tracking systems anticipate environmental changes and adapt to the appropriate time of day. Researchers discovered that the sleep/awake switch exists in the eyes within the intrinsically photosensitive retinal ganglion cells (ipRGC).

Previously, a brain region called the suprachiasmatic nucleus (SCN) was believed to house the master clock that keeps the body on an approximately 24-hour schedule. The current findings give the eyes a more central role in the control of the sleep/awake cycle.

In the nocturnal mice, ipRGC and SCN appear to function similarly, and either could serve as the timekeeper. However, in the diurnal monkeys, the eyes' ipRGC seems to be dominant.

"Considering the long-held view of SCN as the master clock for our circadian rhythms, the idea that the eyes - or particularly ipRGC - are the commander in chief is somewhat surprising," Zhou said.

"But it makes logical sense, as diurnal mammals are visually driven. The eyes not only guide us around during our wakefulness time, but they also dictate when we go to sleep," he said.

Researchers found that the sleep/awake control mechanism differs in monkeys and mice before signalling to the SCN, in the neural circuitry controlling sleep and wakefulness. A novel bifurcation of ipRGC to brain centres produces the inverse sleep/awake patterns for the monkeys and mice.

In mice, ipRGC, via connection to the SCN, signals the animals to sleep during daytime. In monkeys, this stay-put message from the ipRGC-SCN pathway is overpowered by signals from a more dominant clock pathway of ipRGC to a midbrain structure called the superior colliculus that tell the animals to be awake during daytime.

These findings challenge the long-presumed master role of the suprachiasmatic clock in all mammals. At least in monkeys - and possibly in humans - ipRGC may be more important than the suprachiasmatic clock in determining when to be active and when to sleep. The research appears in the journal Molecular Brain.