I am in the drink-more-water camp. The OP is displaying classic signs of dehydration after his rides. The key for me is the need to sleep post-ride, but having a dehydrated brain will produce similar symptoms to a hangover, including the lack of focus.
I find the dehydration symptoms will last 24 hours or more, too. The rehydration process starts on the bike, almost from the start of the ride. It only needs to be in small quantities but around 750ml for every 60 to 90 minutes, more in really hot conditions, slightly less in cooler conditions.
Wind also can play havoc with rehydration strategies -- in moderate temperatures, it can evaporate sweat instiantaneously, resulting in the rider believing they aren't sweating at all, and therefore not taking in sufficient replacement.
If you want to know how much "background" sweating you do, and you live in North America, go and ride now with a windproof shell on, then check the moisture inside when you take it off after the ride.
Feet sweat quite a lot, too, and this is one of the reasons why wind protection in cold weather is essential.