Is his sleep habit healthy or unhealthy?

[davidlee]

My dad, now turning 75, sleeps in the afternoon from 1:00 PM to 3:00 PM (2 hours). Then he sleeps at night, midnight to 6:00 AM (6 hours). Otherwise, he is quite active and alert, physically and mentally.

He says that he can only sleep for 6 hours at night the remaining 2 hours of needed sleep, he gets in the afternoon. Also, he says that this breaks his day into two, and makes the days more enjoyable and productive.

I am concerned about the natural cycles of sleep; he might be missing out the REM sleep, etc.

Are his habits, healthy or unhealthy?