Keep a fixed wake-up time, even on weekends

Cost: No cost involved. Sticking to this on weekends requires a bit of willpower.
In plain terms: For the top 20% of students with the most irregular sleep schedules, their circadian rhythm runs roughly 2.5 hours later than that of the top 20% with the most regular schedules. This is equivalent to living with a persistent jet lag. Moreover, students with more consistent sleep patterns tend to achieve better academic results.
Benefit: Sixty-one university students tracked their sleep patterns over 30 days. The group with the least regular schedules had a circadian rhythm that lagged by about 2.5 hours compared to the most regular group — specifically, the time when melatonin starts secreting was 00:08 versus 21:32 for the latter. The period when they felt most sleepy also occurred roughly 1.8 hours later. Additionally, greater sleep regularity correlated with better academic performance, with a correlation coefficient of r = 0.37.
Evidence grade: B
Sources:Phillips et al. (2017). Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Scientific Reports. https://doi.org/10.1038/s41598-017-03171-4
Notes: This is the sole study on this topic, with a relatively small sample size and no control groups. Researchers estimate that these circadian differences stem mainly from varying daily exposure to natural light. Therefore, maintaining a fixed wake-up time works best when paired with exposure to morning sunlight. For tips on how to recover after occasional late nights, see Section 2, Item 39: the recommended approach is to go to bed earlier that very night rather than sleeping in until noon the following day.