Well, it MIGHT have to do with what your “body clock” wants vs what your “social clock” wants. The difference between the two is what Prof. Till Roenneberg and his colleagues refer to as “social jet lag.” He says that our daily lives are controlled by three different clocks:
Our Body Clock
Our Earth’s Rotation Clock (Day/Night)
Our Social Clock
Because we live inside, screened from sunlight, we awaken later and later. Also, we may fall asleep with our body clock, but awaken by our social alarm clock before our body clock was ready. On work days we tend to not get enough sleep and on weekends we may sleep late to make up for the deprivation.