The fixed sleep time does more than the early wake time
Of everything in dinacharya, the single instruction with the strongest modern backing is the least romantic one: sleep and get up at the same hour every day, including Sunday.
In the texts
Dinacharya sets waking in the last watch of the night and treats the hour of rising as the anchor of the whole day, not a matter of preference.
In the trials
Regularity of sleep timing is consistently associated with better sleep quality and mood in midlife women; consistency shows up as more important than duration alone in much of the current work.