Strange things happen to sleep as the body ages. For humans, getting older can mean waking up over and over again or shifting to a much earlier wake-up time. In elderly mice, the region in the brain that directs the circadian clock—the suprachiasmatic nucleus—can go off the rails, as the 10,000 cells that make it up, which normally fire all together, lose touch and fall out of sync.
In a new set of experiments published in the journal Neurobiology of Aging, researchers put young and old mice through a battery of tests to compare their responses to jet lag. They found that elderly mice are far less flexible than young mice, and the reason appears to lie in a particular deficit in the brain as they age.
The circadian clock is reset by light. When we open our eyes after sleeping, the sun activates the suprachiasmatic nucleus and the clock starts over again. Leave mice—or, for that matter, people—in the dark, and at first, they’ll get up and go to sleep at roughly the same time as usual. But let it go long enough and their days will start to get longer, as their internal clock drifts without any external signals from the sun.
In these new experiments, Gurprit Lall, a professor of pharmacology at the University of Kent, and colleagues kept young mice and old mice in the dark for 10 days, recording the hours both groups spent on their exercise wheels to keep track of when they were awake. Then the researchers exposed the mice to 15 minutes of light, setting their clocks ahead as if they’d just gotten off a plane. The young mice responded easily, and adjusted their period of activity later by an hour. The older mice, however, responded much less, moving only about half an hour.