Nobody knows for certain what causes muscle cramps/spasms, and what works for some folks may not work for others. Check the research papers on PubMed and elsewhere. There are no definitive conclusions. That's why there are so many of us swearing by or at our personal favorite magic potions.
That said, creatine and magnesium lactate seem to work for me, in addition to the usual stuff -- mostly, just being properly hydrated and riding within my limits if I want to finish a longer ride.
I usually add creatine powder to my morning energy drink potion. Either Six Star creatine or their whey protein which also contains creatine. Seems to help reduce that burning muscle fatigue sensation, and it clears quicker after reading that point of muscles-on-fire.
Magnesium lactate is available in capsules but I usually open the capsule and add the powder to my usual pre-ride/workout mix: cold coffee, whey protein powder with BCAAs, electrolytes, extra calcium, Vitamin D, etc. (I have early onset bone density loss problems so I take all the help I can get), mag lactate, and whatever other fancypants supplement has caught my eye from the Bulk Supplements or other catalog. Currently it's concentrated spinach powder. Next month, who knows.
Anyway, I have far fewer muscle spasms during rides or while trying to sleep.
I also use a roller on my legs after every ride. And I can tell when I forget -- my legs will feel dead the next day. Some folks use foam rollers but I find those awkward. I use a heavy marble rolling pin that I already had in the kitchen. A heavy duty percussion massager would work as well.