Excerpt from:
Circadiana: Everything You Always Wanted To Know About Sleep (But Were Too Afraid To Ask)
Until not long ago, just about until electricity became ubiquitous, humans used to have a sleep pattern quite different from what we consider “normal” today. At dusk you go to sleep, at some point in the middle of the night you wake up for an hour or two, then fall asleep again until dawn. Thus there are two events of falling asleep and two events of waking up every night (plus,
perhaps, a short nap in the afternoon). As indigenous people today, as well as people in non-electrified rural areas of the world, still follow this pattern, it is likely that our ancestors did, too.The bimodal sleep pattern was first seen in laboratory animals (various birds, lizards and mammals) in the 1950s, 60s and 70s, i.e, before everyone moved their research to mice and rats who have erratic (un-consolidated) sleep patterns. The research on humans kept in constant conditions, as well as field work in primitive communities (including non-electrified rural places in what is otherwise considered the First World) confirmed the bimodality of sleep in humans, particularly in winter.
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Popping melatonin pills is one of the latest crazes. Melatonin failed as a sleeping pill and its uses as a scavenger of free radicals are dubious at best. It can shift one’s clock, though (rebeldoctor.blogspot.com…. However, it cannot help against jet-lag or effects of shift-work (shift-lag) as melatonin is likely to shift only the main brain pacemaker in the suprachiasmatic nuclei. The problem with jet-lag and shift-lag is dissociation of rhythms between cells in different tissues, i.e., your brain clock may resynchornize to the new time-zone/schedule in a couple of days, the clocks in your heart and lungs in a week, and in your stomach and liver in a month. In the meantime, everything in your body is desynchronized and you feel really bad. If you keep changing your work shift over and over again, you never get to achieve complete synchronization, leading to long-term effects on health, including significant rise in heart attacks, stomach ulcers, and breast cancer.

Well, intercontinental flight is here to stay, and some shift-work is neccessary for the modern society to survive. It is now understood that some people (chronotypes) adjust to night-shifts and even properly executed (non-rapid, phase-delaying) rotating shifts, better than others. People have always tried to self-select for various schedules, yet it has recently started to enter the corporate consciousness that forcing employees into unwanted shifts has negative effects on productivity and safety, thus bottom line. See Chernobyl, Bhopal, Exxon Valdese and Three Mile Island accidents – all caused by sober but sleepy people at about 3am, just like thousands of traffic accidents every year.