Scientists have explained why some animals sleep at night and are active during the day, while others do the reverse.
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.
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.
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.
