But the circumstantial realities of old age change that, to a significant extent. "Social jet lag" -- the phenomenon through which our natural circadian rhythms are undermined by rigidly collective social schedules -- is less of a factor for people who aren't (generally) working and whose daily routines aren't (generally) governed by strict itineraries. It is less of a factor, in other words, for people who are relatively unreliant on technology. Retirees can sleep when they need to, wake when they want to, and generally obey the whims of their bodies much more readily than younger people can.

And that change in the way older people live also affects -- potentially, probabilistically -- the way they die. Because, just as circadian rhythms regulate things like preferred sleep periods and the time of peak cognitive performance, they also regulate the times during which we're most likely to experience an acute medical event like a stroke or heart attack. As study co-author Clifford Saper -- who is also the James Jackson Putnam Professor of Neurology and Neuroscience at Harvard Medical School, and also the chairman of the Beth Israel Deaconess Medical Center Department of Neurology -- explained to me over email: There is a "biological clock ticking in each of us."

So the technological freedom that comes with people's retirement can actually end up bringing a kind of cruel regularity to their deaths. What Saper and his co-author, Andrew Lim, realized through their research is that there seems to be one DNA sequence that determines, essentially, how each of us relates to time itself. And data analysis -- poring through 15 years' worth of sleep and death patterns collected from subjects in an unrelated sleep study -- helped them to make the realization.

Saper elaborates:

What we found was a "single nucleotide polymorphism" or SNP where about 60% of chromosomes have an adenosine (A) and 40% have a guanine (G) in the DNA code. Because you have two copies of each chromosome (one from each parent), 36% of people have two A's (AA), 16% have two G's (GG), and 48% have one of each (AG). When we looked at a group of older but healthy people (all over 65, so mostly retired) we found that the AA's tend to wake up about 1 hr before the GG's, with the AG's in the middle. What we think this means is that the AA's have somewhat higher amounts of a protein called Period1 in their cells, which we think causes their biological clock to run a bit faster. On a daily basis, what this means is that they would tend to get up a bit earlier each day. If this persisted (getting up earlier and earlier each day) they would soon be getting up in the middle of the night, and going to bed before dinnertime. This obviously would cut down on their ability to socialize with other people. So, we think that the AA's adjust their timing a bit each day, to stay at a "socially acceptably early" schedule, which is on average about an hour earlier than GG's.

That makes sense. And then, since circadian rhythms control wakefulness -- alertness, blood pressure, heart efficiency -- it stands to reason that the flip side could be true, as well: that the rhythms that stimulate human activity could also stimulate its end.