What Is The Reason For Muscle Stiffness In The Thigh?

I'm 21/m and recently i have had muscle stiffness in my thighs when i wake up in the morning. It continues for most of the day. Stretching helps but doesn't make the stiffness go away, same for exercise. I have no way of knowing for sure since i live alone but i think i don't move at all at night. Is this a serious problem?