The silence, I'm told, is way worse than the snoring. In the middle of the night, you go quiet for a while. Your chest heaves. Nada. Your body tries again. Still nothing. Then, if you're lucky, your brain kicks in and sends out the alarm: without oxygen, it will starve. So your reflexes get your body to rouse; there's a snuffling, wheezing and then a big intake of breath. And then back to normal breathing—or more snoring—until the cycle starts again. And all the while, you're fast asleep, blissfully unaware that anything is going on.

This is called sleep apnea. It happens when your nasal passages or throat gets blocked during sleep and your breathing is badly interrupted. It's often, but not always, correlated with carrying too much weight, and it occurs more in men than in women. Most people who have it have no idea. I sure didn't. But eventually my long-suffering boyfriend insisted I go to a sleep clinic to get myself examined.

With all my spluttering and huffing and puffing, he was sometimes alarmed that I might not make it through the night. Sleeping next to

After a brief educational video, I was placed in a bed with a forest of small wires attached to sensors all over my head and face and even legs. The staff trained a video camera on me and fitted me with a brace to measure my chest movement. Then I was told to relax and fall asleep. Yeah, right. Eventually I did. And then halfway through the night, a nurse came in and put a special mask on my face. It looks like a respirator, which is what it is. The CPAP (an acronym for continuous positive airway pressure) machine is designed to blow air at a steady pressure into your airways to keep you breathing regularly. Hooked up, I drifted off again. The nurse measured my sleep patterns remotely and varied the air pressure in the CPAP to maximize my sleep.

In the morning I had a truly unexpected sensation. The nurse woke me up at 5:45, a time of day I hadn't really experienced since high school. And I felt fine. More than fine, actually. I felt like a 10-year-old after a cappuccino. Since I normally take a couple of hours after I wake up (around 10 a.m.) to arrive at even moderate alertness, I was stunned. What had happened? A week later, I got my results from the sleep clinic. Without the CPAP, I had stopped breathing on average 38 times an hour. I had got absolutely no Stage 4 sleep, the kind that really refreshes your mind and body. With the CPAP machine, I breathed normally, and 17% of my sleep was Stage 4.

No wonder I felt better. And that was after only four hours of good sleep.

How long had I been laboring against apnea-produced tiredness, irritability and lack of stamina? I have no idea. The trouble with apnea is that it's almost impossible to self-diagnose (because you are unconscious when it happens), and its symptoms are easily explained by other possible factors. Nodding off in the afternoon? Too much work. Snuffling at night? Just a snoring problem. Irritability? A character problem. Constant need for naps? Idleness.

Before you know it, you're addicted to Red Bull and constantly grumpy. Sound like you?

My first few nights using the CPAP at home were not quite so dramatic as the clinic stay. It takes a while to get used to the thing, as you can imagine. At first, when you clasp the oh-so-attractive contraption to your head with its Velcro straps, you feel like Jacques Cousteau at a slumber party. But you get used to it. And as each day passed, I felt energy gaining in my mind and body. My postapnea life is just beginning. And for the first time in a long while, I'm raring to go.

• Andrew Sullivan