Another reason why exercise keeps you younger?

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Reading through a copy of the New Scientist from a few weeks back (2 February edition), I was struck by the comment in an article on the effects of sleep on the human body by Nancy Wesensten, a psychologist at the Walter Reed Army Institute of Research in Maryland:

Sleeping deteriorates like everything else does as you age… People have more difficulty falling asleep, and that could account for the cognitive decline we see in normal ageing.

Until I started a vigorous exercise regime about 16 months ago, I really did find it difficult to fall asleep. Since then, while I don’t have my partner’s ability to more or less doze off as soon as my head hits the pillow, I generally no longer have a problem.

I have often seen claims made for exercise as a means of maintaining mental acuity – perhaps there is some substance to those claims and this is the reason?