I have sleep apnea and I know a couple of other guys in Tenth Planet do as well (and there's a few more probably undiagnosed). While sleep apnea is commonly associated with middle-aged overweight people, it also hits athletic types with muscular necks (like people who are constantly fending off darces and what not).
Obstructive sleep apnea is when the muscles of your throat collapse, blocking your airway. You wake yourself up (even if you don't realize it) and this disrupts your sleep cycle so you don't get the proper rest. This is often associated with snoring. There's also central sleep apnea and that has something to do with the brain. Anything that disrupts your sleep can impact your health in a number of ways. At the least your energy levels decrease and in extreme circumstances you could have a heart attack or a stroke. So take it seriously, you could be adding years to your life and drastically improving the quality of those years.
The sleep test is basically an overnight stay at a clinic where you sleep with wires attached to your head and your nose to monitor your breathing. The sleep doctor will make a diagnosis based on the results. If you have a very mild case, they might just tell you to start sleeping on your side. If you have a mild to medium case, they may offer you a mouth guard that will push your jaw forward so your throat can't collapse. The surgery is an option but the muscles grow back so it's not as favored (at least, I think that's why they aren't so hot on it). Severe cases need the mask and machine.
If you are offered the mask and machine, ask about the mouth guard option. I was offered the mask but requested the mouth guard. The doctor agreed that would do in my case and it's worked great. The problem with mask and machine that forces oxygen into your lungs is that you're unlikely to get laid unless you hang out at a Stars Wars convention and look for chicks with a thing for Darth Vader (or if you are married).
Don't assume you don't have an energy problem. It's kind of like glasses. You don't realize you have a problem until you see what the rest of the world sees.
I also have narcolepsy. God just really wants me to take naps and I'm cool with that.