Mark Ellis recently met an ex-pupil from a primary school where he regularly did supply teaching. "Do you still fall asleep in the classroom, Sir?" the child asked him. "I remember one day you kept falling against the blackboard and you couldn't stand up. We thought you were drunk."
These days, Mark (not his real name) is very wide awake. But for years he fell asleep not only in the classroom (story-reading was risky) but also in meetings, over meals, while playing bridge with friends, even in the middle of a live football match. Every day he went into work feeling exhausted.
"Other members of staff weren't very sympathetic. They would see me yawning or feeling groggy and miserable, and they would say, 'Why don't you go to bed earlier?' Whereas I had been to bed early, and I'd slept all evening on the sofa."
At lunchtime, he would steal away to sleep in his car. Still he dozed during afternoon lessons. He would splash his face with cold water from the classroom sink, then leave for home, taking his marking with him. "There were marks on the children's books where my pen slipped because I had fallen asleep."
He and his partner, Elaine, also a teacher, assumed his sleep problem was due to stress. Their GP agreed and signed him off work. Pursuing rest and relaxation, the couple went on holiday to Cornwall. As usual, they booked separate bedrooms, because for years Elaine had been unable to sleep through Mark's snoring. But on the last night the hotel was fully booked and they had to share. Elaine lay awake all night, not only deafened by Mark's snoring, but terrified by the new discovery that between snores he seemed to stop breathing altogether.
A few days later, coincidentally, a friend sent them a newspaper cutting.
(The same friend who had sat next to Mark while he slept in the stands among 30,000 cheering football fans.) It described the experience of several people with obstructive sleep apnoea: a condition in which the sufferer's soft palate and tongue muscles relax too much during sleep, blocking the passage of air into the lungs. As the oxygen supply to the sleeper's brain drops, it feels suffocated and wakes the sleeper up.
Mark returned to his GP, who sent him to a hospital sleep clinic. Doctors there wired him up to a computer which would measure his heart rate, oxygen, blood pressure and snoring levels while he slept. After he got up seven hours later, feeling as ghastly as ever, they told him what the computer reading revealed. During his "sleep", Mark had in fact woken up around 300 times - more than once every two minutes throughout the night.
His blood pressure, heart rate and oxygen, instead of remaining steady, were a wild zigzag of suffocation and revival. He had never had time to get any deep sleep. And his snoring had peaked at 98 decibels - for which health inspectors would insist anyone nearby must wear earmuffs.
Fortunately for Mark, the doctors could also offer a recently invented and ingenious solution. It came in the form of a soft triangular mask, attached by a flexible tube to an airblower - a kind of vacuum cleaner in reverse.
He puts on the mask when he goes to bed, and the steady stream of air passing through the tube and into his nose keeps his airways open throughout the night. "The first time I wore it it was a bit uncomfortable," he said. "Now I just put it on and go to sleep.
"Within a week of wearing it my headaches had stopped: I used to get terrible headaches and the doctors said that was due to the oxygen starvation. I was back to normal after about three weeks. Now I get to the end of the day and I have lots of energy. I can even do paperwork."
Mark now wants to restart his career and is supply teaching while looking for a responsibility post in a primary school. He must wear his sleeping mask forever. It means that during the day he can read, watch TV, run an after-school club, exercise, play bridge, go to football matches - have a life, in fact.
Looking back, the five years he spent without sleep horrify him. "I really did think I was cracking up. I was narky with kids, and I never used to be like that. My confidence had ebbed away, and I was just getting through the day. Now I'm back to the person I was before."