Hi Magneto. I would first ask; are you getting enough sleep? Exercise is one of my priorities and I do it first thing in the morning when I’m fresh. But I make the choice to turn the the TV or computer off at the same time each night so as to get my 8 hours sleep. Also, sometimes when we jump into something, we try to do too much too soon. Endurance takes a while to build so your swimming should start off at an easy enough pace so that you are not exhausted when finished. If you are new to exercise then a gradual increase is the best way. I hope this helps and good luck!
Jim