It certainly is biology. Your circadian rhythm is strongly influenced by the timing of day (well, blue) light being received by your eyes. It also shifts natura

It certainly is biology. Your circadian rhythm is strongly influenced by the timing of day (well, blue) light being received by your eyes. It also shifts naturally as you age - for example, in your teenage years you tend to fall asleep and wake up later. There are also interesting effects like the significant differences in school performance between students living at the eastern edge of a time zone vs living at the western.