I sometimes get short head pains during the day, probably due to tmj. but i always get the pains when i lie down, they stop when i sit up. how come?

Possibilities?: If you suspect TMJ dysfunction, i would first recommend a full exam by a dentist with experience in treating tmd/tmj. He can tell you if your TMJ is part of the problem including the cause of the head pains. Another possibility from your symptoms could be a sinus issue. When a pain in your head occurs and goes away due to head position, that can involve the sinuses. Good luck.

Positionsal pain : with TMJ can be due to inflamed capsule or retro discal tissues in the TMJ. Pressure of jaw falling back or being pushed by hand or bed pressures may impinge those inflamed tissues.

May not be TMJ: May not be TMJ. Perhaps your clenching or grinding when you lay down. See a TMJ expert.