A very small area of my tongue indents on its own, like spasm,then slowly releases.it's always the same spot, and it can last for hours.it doesn't hurt but i feel it and i can see it.like indent appears then releases multiple times. what causes this?

I’ve never heard of this before but consider it may be a simple muscle twitch that may mean you need more sleep or a mineral such as potassium.

It is possible that spasms in your tongue may be caused by inadequate space in your jaw. Your tongue should rest comfortably at the roof of your mouth without scalloped edges, fissures or cramping. Read the book Six Foot Tiger, Three Foot Cage by Dr. Felix Liao. You may want to see a dentist or orthodontist & get a referral to a Oral Myofunctional Therapist who can prescribe exercises & treat.