I have heard this argument but also the argument that the reason people woke up in the middle of the night was to stoke a fire (or other heating device). Person

I have heard this argument but also the argument that the reason people woke up in the middle of the night was to stoke a fire (or other heating device). Personally I am not entirely convinced. I am a believer in some people being more tuned to night owls or morning people however (not that it is super firmly set in stone - just a bit of a trend). There is also the theory that this benefited smaller societies since having a few people awake at all hours while others slept would have been good for noticing threats. Combining these two thoughts actually may be the answer - perhaps we are not biphasic but that few hour window in the middle of the night was driven by the night owls and the morning peoples drive to socialize with one another (sex, eating, etc as you say)