We wouldn't say '8 hours one day'. We'd say '8 hours a day', '8 hours per day', or '8 hours every day' if we wanted to emphasize the habitualness. If it's a one-off situation, you could say something like 'I slept 15 hours in one day'.

"I always sleep 8 hours one day" is incorrect. As Bill said, if it is your daily routine to sleep for 8 hours, then you may say " I always sleep 8 hours a day/per day." "One day" can be used if you are talking about specific day in past. For example: One day I slept for 8 hours only. "One day" is also used when we want to talk about unknown day in future. For example: One day he will succeed.

"a day" can also mean "within a day" and "every day"

One day, i'll sucessed. Because i just sleep 6 hours a day, and i take 4 hours to upgrad myself per day.

Bill's explained the difference in your example well. The general difference is that "one day" means one certain (or even specific) day, and "a day" means "any day" or "any given day".