[mapt]

"Going to bed early" as in, substantially earlier than the day before?

There are a lot of cues that change when you get tired, but the body's internal rhythms should be expected to work on roughly a 24h cycle, and resist attempts to disrupt any cycle that's been followed for the last few days. It seems to work towards sufficiency of sleep about as strongly as it works towards regularity.

And we've engineered a week in which we're expected to 'sleep in', two days out of seven.