I have read some 2000+ research papers on sleep and written a book on the subject. So, you would imagine that I sleep well.  I do sleep well – mostly.  I certainly sleep much better than I did a couple of years back before I did the research for my book.  But I do not sleep well every night.  Most of the time I can pin the cause of poor sleep – a late cup of tea, a heavy dinner or some such violation of the rules governing good sleep. But, every now and then, I will have a night of poor sleep for no apparent reason.
My poor sleep could simply be attributed to a consequence of ageing (I am 58). Babies sleep brilliantly – they can sleep through the sound of a jackhammer. Teenagers often sleep poorly – but that is a self-inflicted social jet lag resulting from a “late to bed, late to awaken” chronotype that does not match with school timings. As young adults, we may have our occasional night of poor sleep – often due to the concerns and worries of raising a young family while simultaneously negotiating the corporate ladder.
But in our later 40’s and 50’s, poor sleep starts becoming frequent. Why? – No one knows. It is possible that evolution has intentionally engineered light sleep as we grow older. If the older folks in the tribe slept lightly they could alert the deeper sleeping youngsters in the event of danger – say an attack by a predator or an enemy tribe. In any case, since they have crossed their reproductive age the health effects of poor sleep may be less consequential for these older folks. Now that we have no predators and can lock our bedroom doors before we sleep, this evolutionary vestige is maladaptive.
It is the lucky few among us who can enjoy peaceful rest night after night. For the rest of us, it is a heavy slog working through the numerous permutations and combinations of factors that affect our sleep. But I can state from personal experience that the slog is well worth the effort. Nowadays my concern over an occasional night of restlessness is leavened by the thought that good sleep is just around the corner and I can tip the odds in favour of it happening!