Quantity: sleep at least 7 hours per day.
Quality: sleep in a dark and cold room (15-19 degrees Celsius).
Timing: go to bed at the same time every night.
Habit stacking
Level 1 (Q): sleep at least 7 hours per day.
Level 2 (Q+Q): sleep at least 7 hours per day in a dark and cold room.
Level 3 (Q+Q+T): sleep at least 7 hours per day in a dark and cold room at a consistent time.