People can lose anywhere from 200mg to 2000mg of sodium per 32-oz of sweat. Gatorade contains 160mg of sodium per 32-oz. If you're a heavy or extra salty sweater, you probably need more than you're replacing.

Regular v8 will replace sodium at a high rate (4x as much as Gatorade). Pedialyte or Gatorlyte are a good middle ground. Or you can even just add a bit of table salt (1/4 tsp is about 500mg of sodium) to your sports drink.

Magnesium supplements before bed are also very helpful as you've discovered. Magnesium Glycinate is great for replacing Magnesium in the bloodstream without a laxative effect. Magnesium L-threonate is good for calming an overactive brain and/or nerves, because it passes the blood-brain barrier. It can also have side-effects, so use according to directions if you feel you need it.

I've competed in endurance sports (marathons, triathlons, and every type of bike racing) for many years, and it took me a while to realize that I needed a LOT more sodium in my sports drinks than the average person (and a lot more to drink). Once I got everything dialed in, workouts and races got a whole lot easier, as did recovery and sleep. I often consume 3000+ mg of sodium on a long bike ride in the summer (4+ hrs) for optimal performance and recovery. But I also recognize that I'm not the norm. Good luck! Sounds like you're well on your way to getting it figured out.