Everybody is aware that each person needs to sleep well at night in order for the body to be able to function properly the next day. What many people are not aware of is that the night sleep has a very great spiritual need also.
As such, when a person goes to sleep at night, they should have in mind that even while sleeping they are serving the Borei 'Olam (the Creator of the world). That is why Rabbothenu z"l instituted the Beracha (blessing) of Hammappil when a person goes to sleep at night.
Sleeping during the day, however, is a different matter. We are told that sleeping during the day is harmful to the righteous and beneficial to the wicked. (This is obviously not referring to a case where someone is unwell or has some very great need to nap during the day). But what we do see from this is the importance of getting a good night's sleep so that we can function normally during the day, both for one's spiritual as well as physical well being.
On Shabbath, however, sleep during the day is beneficial to the righteous also.
(See Ben Ish Hai, Shanah Aleph, Parashath Piqudei, Oth Heth)
