Erika has good thoughts and I'll add one more, which may or may not be appropriate.

Erika advises magnesium 500 mg a day. I have to take 1000 mg a day to get muscles to relax and stop jerking and spasming at night. I take 500 at lunch and 500 at dinner. I also take Calcium between meals, never taking more cal than mag.

As Erika says, stretching and massaging the tense muscles (muscles in spasm) helps temporarily. I have to do this almost every night. I have a regular routine which lasts up to 45 minutes if the knots in my legs refuse to relax with stretching and self-massage.
I have developed more arm strength than I had by (desperately) rubbing the knots out of my legs when I lie down to sleep, if there are serious knots there. Sometimes it takes as long as 45 minutes, but usually a much shorter time is needed to get them to relax enough for sleep to happen. The stretching part is always needed too, as one loses the stretch in the 24 hour period, and one has to do it all over again.

What I said about rubbing out the knots may not be appropriate for everyone. Perhaps it would depend on how spastic you are. If the knots CANNOT be rubbed out, then they can't.

One thing I do during the day (here adding another exercise) is to deliberately tense muscles for a few seconds and let them go, concentrating on the muscle which I'm working on to let go fully. This helps during the day, especially if you have something which causes a tensed area, such as the neck, after a minor whiplash. By a minor whiplash, I mean the type of jarring which occurs when driving over a speed bump too fast. I try to go as slowly as possible over bumps, but sometimes another car is behind you, ad you can't slow much, so a jarring and minor whiplash occurs. I always immediately tense and release my neck muscles after such an occurrence, and it has reduced my need for a chiropractor, which I would say is success.