I can’t comment much on the physiology (and I’d be happy to receive more info about this), but there are a some things that come to mind.
The Buddha often talked about mental and bodily tranquility or ease that arise when cultivating deep serenity and wakefulness. The mind comes to rest from agitating thought processes, and the body becomes very relaxed, easeful, full yet light. The breath also becomes very subtle and calm. All of this seems to have an affect on the nervous system — putting the body into ‘rest & digest’ mode as opposed to an anxious ‘fight or flight’ mode that comes when the breath and body are tight, and energy lifted up near the chest and head due to mental activity and tension.
So the mind is very still and restful, and there is very low tension on the body / nervous system. I’m not sure how the quieted mental activity relates to sleep physiologically, but just how ‘sleeping on’ something allows ones mind to return refreshed and re-juvenated, samādhi has the same effect: when mental activity does return and one engages in it, the mind and body are deeply refreshed and renewed.
At the same time, and on the other end of the spectrum, mental and physical drowsiness drop away. These are of course hindrances to samādhi, and so one cultivating it will be free of them. So the mind and body will not feel the need to sleep very much, and will naturally incline to a clear, wakeful presence. This is opposed to people who deprive themselves of sleep, all the while feeling lethargic, dull, heavy, and drowsy. In that case, the body and mind clearly alert us that they lack and need sleep, as opposed to times of deep cultivation where they say the opposite.
I believe the Buddha was recommending and proposing a middle way — one that must be evaluated against what you may hear in modern Buddhist circles. If we look at a key discourse addressing drowsiness and sleep (AN 7.61), the Buddha tells Mahāmogallānā (who was struggling with sleepiness) that if he cannot resolve his sleepiness by adjusting his mind/body, then he should sleep until he is rested enough. This indicates that it is okay to sleep if the body-mind really needs it; but we don’t indulge in it or look to it immediately, because from the perspective of proper cultivation, the need for lots of sleep will fall away.
In the same sutta, the Buddha talks about the importance of maintaining restraint, inner calm, and being in secluded environments for deep practice; I believe this is related to the theme before. If the mind is active, caught up in things, and far from samādhi, it will need sleep and become drowsy; if it is wakeful yet still, the body and mind can unify and need for sleep will diminish. So he’s recommending external conditions outside of the seated session for developing wakefulness.
If we are not at this point yet, again, we don’t pretend we are and ignore our body/mind — we adjust accordingly, without going towards over indulgence. In fact, I found the article you shared interesting in saying that over indulgence in sleep has clear correlation to increased mortality. The recommended amount of, say, 6.5-7 hours is relatively moderate, and getting slightly less is still less dangerous than over-indulgence — especially if we factor in what I mentioned above about practice.
As far as the ‘watches of the night,’ it’s often interpreted to mean 4 hour periods. I’m not entirely convinced. The first watch of the night essentially means the evening. The second is the real ‘night.’ The third is the early morning before dawn — the time when monastics traditionally go alms-round (referenced in the suttas). So if the evening goes from around 6-10PM, and the dawn is around 4-5AM, we could see a rough time of about 10PM-4AM, of 6 hours of sleep, which is reasonable both in terms of bodily health and dawn-time. Even if the ‘third watch’ begins slightly earlier, the suttas say one gets up during this third watch but before alms-time, so it could be any time in the middle so long as the night is not over. They weren’t using military grade alarm clocks in Ancient India.
The point is not to go to bed early just because the day is closing, and not to stay sleeping too late so as to miss out on quiet time for cultivation before alms-round. We don’t turn the mind away from reality by taking refuge in sleep; but we don’t deny our body it’s physical necessities. The same is true for food, clothing, shelter, and medicine in the Buddha’s teaching.
I hope this is beneficial 
That’s about most of the info on wakefulness that comes to mind, so it should be relatively comprehensive. Mettā.
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). So we get 2 hours of sleep, all night meditations etc…
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