I have a 3 yr old bassett beagle mix. I noticed something different this morning. When he relaxes
His whole body seems to have a spasm. What
Could it be?

Is he dreaming? When my dogs are sleeping they howl and whine, and their whole bodies twitch. I guess they are chasing those rabbits in their dreams. Very uncomfortable when you are sharing a bed with a 120 lb dog.

Dogs have different phases of sleep, just like humans do. The only difference is in the length of time. Our sleep cycles tend to be longer, but like people, dogs go through slow wave sleep (SWS) and rapid eye movement (REM) sleep. All that dreaming happens in the REM sleep cycle.

Sounds to me like normal muscle tremors while he sleeps. Although seizures could be a possibility, but that is something you would need to talk to your vet about ;)