Out of tune bass guitar. Please read detail?

Okay after i buy my bass guitar, i tuned it to standard tuning and i played for it for about 2 hours. After that, i relax and sleep for a while. After i wake up, my bass is suddenly out of tune and i re tuned it and played. then i put the bass for an hour and when i get back to play it, it is suddenly out of tune again. This happens everyday. Is this normal? Any helpful tips? Tnx!!