There are a number of possibilities, but more detail is necessary. Where exactly on the gauge is your needle during the day when the fan activates? Where when the fan deactivates? And at night?
Have you noticed anything else unusual, like fluid loss, power loss, MPG loss?
It is normal for the needle to move all the way to the M of NORM before the fan activates to bring it back down between R & M.
Also, check your coolant level, radiator cap, & overflow hose for blockage. Perhaps you've air in the system.