The curious incident of the clock in the night time

Ok this is really weird. every night, my clock slows down 15 mins during my sleep

When i wake up every day, the time somehow slows down by 15 mins. I set the time back every morning and the same thing happens over again for the whole week !

Freaky.

2 explanations

1) the clock battery is failing ONLY at night

2) I've experienced space-time bending (perhaps alien abduction ?)

either way, it's still weird.