> Your 30+ year old body
Hey, wake up call, not just to parent commenter but anyone.
If you feel this way in your 30's chances are good you should probably spend more time focusing on your physical health. Start getting more exercise and working on your diet.
You don't need to go crazy, just make small improvements and try to stick with them as much as you can. Start simple: Go for a short walk in the morning when you get up. Cut back on refined sugars, alcohol and sugary drinks.
I'm in my early 30's and my capacity to push hard and be durable is stronger than ever. But I made small but steady steps on my diet and kept up a modicum of conditioning throughout my 20's. I love backpacking, so "durability" is something I'm very interested in.
For example, this weekend I was easily able to stay up and game with friends in their early to mid 20's(UFC 5 release) until the wee hours of the morning. I then slept curled up in a ball on the couch(it was cold and that position kept me warm) and bounced up first thing in the morning after like 3 hours of sleep feeling great.
I don't do things like that often because it runs counter to being healthy. Sleep is when your body repairs itself, and it is constantly taking damage. My experience also is that when you are young, you are just less in-tune with your body. By the time you are 30 most people are actually able to recognize how detrimental sleep deeprivation is in the moment.