&quot;Going to bed early&quot; as in, substantially earlier than the day before?<p>There are a lot of cues that change when you get tired, but the body&#x27;s internal rhythms should be expected to work on roughly a 24h cycle, and resist attempts to disrupt any cycle that&#x27;s been followed for the last few days.  It seems to work towards sufficiency of sleep about as strongly as it works towards regularity.<p>And we&#x27;ve engineered a week in which we&#x27;re expected to &#x27;sleep in&#x27;, two days out of seven.