It would seem that it doesn’t matter when you sleep, as long as you will get enough hours. But there are reasons why this is not fully true.
First was outlined in Circadian rhythm chapter. Our sleeping and waking hours are guided by the ebb and flow of adenosine, melatonin and cortisol during the day. Sleep onset is short (= it’s easier to fall asleep) when the adenosine and melatonin are high, and it will probably be very difficult to fall asleep if none of them is present.
Second reason is the extension of the impact of different sleep phases. While we usually get all types of sleep throughout the night, based on latest research our bodies focus more on deep sleep during the earlier hours of the night, and then in second half we focus more on REM. If we push our bedtime a few hours later, we might miss on opportunity to get enough deep sleep, and if we need to wake up too early we are compromising on REM.