Sleep on a schedule you can defend
Sleep is the habit most likely to be traded away for work, and it is the one that most directly shapes how you feel. A fixed wake time is easier to hold than a fixed bedtime, because it anchors the rest.
Screens, late caffeine, and heavy meals close to bed all push sleep later. If your schedule genuinely varies, protect the hours you can control rather than chasing a perfect eight — the point is a floor, not a target you miss every week.