Dont know if anyone else has this but ive been playing guitar for 10 years and have been practicing with breaks consistently now this last month and for some reason nights and darkness at like 7:0 PM my hands and body are “relaxed” instead of in the day. Their somewhat synced at night and it’s where I practice the most at my best. I’ve also been trying to bake relaxation stuff for my whole body reading some books. I used to be able to have that loose feeling in the day and mornings when I was younger I’m 24 now but for some reason night is when I’m decently relaxed. I’m trying to increase my practice time a little bit and try to do day ppracticefor short periods and do longer periods of practice at night while taking breaks to not have tendonitis or carpooltunnel h
Hmm, could be that you’re relaxed because it’s the end of the day?
When you play during the day, do you have as long of a warm up as you do at night?
Mostly just now I’ve been trying to do stretching and warm ups in the morning. I dont touch my guitar warming up because then hand sync becomes weird and it takes a decent amount kf time to get it to that stage like I do at night.
The Trigger Point Therapy Workbook
I love this book. A literal map of pain and how to relieve it. If not now, then over time.
Also, try muscle scraping.
You don’t need anything really fancy for scraping. The spine of a butter knife, the cap of a medicine bottle, a spoon, etc. can be used as a scraper. Or go to Amazon and search “Muscle Scraping Tool” and you will get results. Add some kind of lube/moisterizer (I use the same stuff my Physical therapist used Palmer’s Cocoa Butter) and get ready to hear the sound of gravel underneath of a car’s tires.
Another thing is to soak in a warm bath with about 2-3 cups of Epsom Salt for 15 mins. Then switch between a minute of cold water and hot water and do that three times then finish with another cold flush. I do this after a workout and even though science based evidence says it shouldn’t do much to help, I always seem to feel more relaxed and less strained muscles. I’m also adding a cool down period to my workouts as well and that seems to help DOMS (delayed onset muscle soreness)
My bathtub is inside my shower, so I can hop out of the tub of warm water and turn on the cold water with the shower. This is called hydrotherapy. Try and increase the temperature and time your are under the cold water.
I have a Theragun and will use that on my shoulders on down to my fingers to get the blood going.
Maybe some arm compression sleeves?
Maybe google a search for “arm cool down” workout? Maybe there is a guitar one out there?
I encourage you to buy the Trigger Point book. It is something you can use for the rest of your life.
P.S: I realized I put a lot of specific product links in here. I’m not getting paid for anything. I’m just passing on the knowledge I’ve come across and paying it forward. 
Hope this helps.