It is Friday evening and I am lounging comfortably on the sofa. My son is asleep and the bitter memories of the working week are already beginning to drift away towards a hazy, distant horizon. My wife and I are settling down to watch a film on the telly.

There is a period thriller starring Christian Bale which we have been promising each other we would watch for some time, and now, with no need for an early start the next morning, seems to be the perfect opportunity.

However, it quickly becomes clear we have made a slight miscalculation. The film is rather more challenging than I had anticipated. It is a murder mystery with multiple plot twists and diversions thrown up to send the viewer off in the wrong direction. The plot is heavy and complicated and a couple of times I have to stop and rewind, just to ensure that I am properly following each development. It is harder work than I had imagined for lazy weekend viewing.

However, in film, as in everything else, the more you put into it, the greater the reward. I have invested my energy and what passes for my intellectual rigour into this. I have become engrossed in the story and, as the suspense builds, I am expecting a satisfying conclusion followed by the chat when I will tell my wife that I had it all figured out from the very beginning.

I refill the wine glasses as the tale starts to near its conclusion, mentally recapping on what I know, what I think I know, and a plausible list of possible offenders about to be unmasked. I sit up straight as the pace quickens towards its dramatic conclusion. And then….

And then I fall asleep.

I wake up some time later, still sitting upright on the sofa but with saliva drooling down my chin. My wife is now watching an entirely different programme.

‘What…what happened?’ I splutter. ‘What about the film?’

‘Oh, that finished about an hour ago.’

‘But, but, but…why didn’t you wake me?’

‘You looked so peaceful it seemed a shame to disturb you.’

I make some terse inquiries about how the movie ended before I retire to bed upstairs.

The anecdote described here reveals a concern over the inevitably creeping presence of a new and unwanted sign of ageing – the inability to stay awake when I want to.

This is a phenomenon I have been aware of from youth when I used to witness my dad falling asleep while sitting in his chair. It was simply impossible for him to watch any TV programme for a protracted period of time without dozing off. After a few minutes his head would begin to nod forward slowly like an obedient dog and his eyes would gradually close. Occasionally he would sit upright with a start, waking suddenly, but it was a temporary reprieve. Once the process of dozing off had begun there was no going back.

I used to watch this fascinated as a child. I simply could not comprehend how it was possible to fall asleep while sitting in a vertical position. It seemed to me as difficult a task to achieve as playing a Beethoven piano sonata while wearing oven gloves. No matter how tired I became, how often I would yawn, I could sit up for as long as I wanted without succumbing to sleep.

When I did sleep it had to be in bed, and even that often proved be a challenge. My relationship with rest was always complicated and troubled and I suffered at various points in my life from crippling bouts of insomnia. Occasionally, doctors have prescribed medication to help me sleep, but it never seemed to make much difference. Countless hours of my life have been spent fretting and catastrophising lying awake in the darkest part of the night while those around me slept soundly.

And now, compounding the issue of not being able to sleep when I want to, is the new difficulty of being unable to stop myself sleeping when I don’t want to.

My routine of late-night TV viewing has already been significantly curtailed through my inability to get through an hour of screen time before my head starts to drop.

Reading is even more difficult. Previously, I would have devoured several books a month. Now, I am struggling to make it through several pages a month. I simply cannot stay awake for enough time to gain any momentum in a novel.

I am a member of a WhatsApp group which includes a few friends with a shared love of reading. This is one conversation from last week.

Friend: What are you reading now Jonny?

Me: Mythos by Stephen Fry. I started it in October.

Friend: Oh, is it any good?

Me: Too early to say. I’m only 14 pages in.

And the malaise is spreading. A few times I have found myself dropping off when playing with my son. Once we were in the middle of a race in Mario Kart on his Nintendo when he shouted at me for snoozing. On another occasion I dozed off while supposedly supervising him doing his homework. On this occasion he was rather less prompt in waking me.

Last weekend we were sitting on the floor playing a game of chess. At one point I seemed to be in a strong position, only to succumb to a short nap and then wake to discover that I had lost my queen, two knights, a rook, a bishop and three pawns.

I do worry about where it will all end. Will I fall asleep some afternoon during a conversation with my boss? Will I be against a deadline on some major breaking news story only to awake several hours later with my face stuck to the keyboard with saliva?

These are worrying signs. I usually write this column late at night when everyone else in the house has retired. Often, I write it on my laptop while lying in bed. On more than one occasion I have fallen asleep when writing it (perhaps those who read it will emphasise?).

As it stands, I am simply trying to get to the end before I zzzzzzzzzzzzzzzzzzzzzz