"Get up at a fixed time, as early as possible. Eight hours of sleep should be enough. More than this or less than six hours of sleep is usually not healthy."
Rising at a consistent time goes a long way in keeping order in our life, and the more ordered our life is, the more we are able to experience God's peace. Early rising helps get a start on our day and allows us to be less pressed for time.
Waking up at a regular time also allows us to conquer ourself in a little way at the beginning of everyday. Rather that calling that shot ourselves, we submit to the time that our spiritual game plan calls us to
"Conquer yourslf each day from the very first moment, getting up on the dot, at a fixed time, without yielding a single minute to laziness.
If, with God's help, you conquer yourself, you will be well ahead for the rest of the day.
It's so discouraging to find oneself beaten at the first skirmish!" - St. Josemaria, The Way, 191.
Even from a secular perspective, rising at a fixed time is recognized as a helpful piece of managing time and being able to wake up refreshed. This practice will not only help to become spiritual healthy, but even just healthy!