The mornings are gloomy. The evenings are drawing in. Already we are pining for the long bright days of summer. But for anyone with seasonal affective disorder, which bears the unfortunate acronym SAD, the change is far more dramatic. In its most severe form, SAD affects 2 per cent of the population in Northern Europe, and its symptoms include mood swings, binge eating and excessive sleeping.

But the form that interests me is the “sub-syndromal” variety, from which 10 per cent of us are said to suffer, and which means feeling more listless, grumpy, hungry and sleepy than we do in spring and summer — but not ill enough to do anything about it. When I tell a loved one that I have this ailment, he sniffs: “Who doesn’t?” Like most other sub syndromals, I’m not lethargic or depressed enough to bother my GP about it, nor am I putting on the weight that tends to typify SAD sufferers. But I do need nine or ten hours of sleep, and yet I still feel shattered in the mornings. From November until April, I can’t even stay up for Book at Bedtime.

Is my lethargy due to a lack of bright daylight, or does our winter behaviour — sloth, greed and a reluctance to go out — contribute to low moods? Dr Raymond Lam, Professor of Clinical Neuroscience at the University of British Columbia, says: “The physiological symptoms seem to come on first, suggesting it is lack of light, not behaviour. But they can be tied together: the same neurotransmitters that affect sleep and appetite also affect mood.”

So to suppress SAD we need more light. Exposure to a light-emitting box each morning is effective in 85 per cent of cases, according to the Mental Health Foundation. Dr Jan Wise, a consultant psychiatrist in private practice, explains: “Seasonal affective disorder is a circadian rhythm disorder. Everyone has an internal body clock which is re-set by an external alarm. The most powerful cue is daylight. That light needs to hit the back of the eye, suppressing melatonin, the hormone that makes us sleepy.”

So why not just put on all the lights in the house and open all the curtains? Dr Wise explains: “Light is measured in lux, and you need 2,500 lux to suppress melatonin. If your shades are open and your desk lamp is on you might be getting 100 lux.”

In the old days, light-box therapy involved sitting in front of a brightly lit screen for up to two hours. But now there is a £125 device called the Litebook which claims to better mimic bright sunlight. The thinking behind it is that it is not the intensity of light that matters but its wavelength: get that right and you need a blast of only 30 minutes.

But does it work? The first few mornings that I used my Litebook (exploiting its paperback size and rechargeable battery to manage five minutes by the bedside, five minutes in the bathroom, five minutes by the make-up table and 15 minutes in the kitchen), I felt crosseyed under its rather weird glare and found it tricky to get the angle right.

“What is that thing?” asked my husband, son and daughter at various intervals.

“Its a Doctor Who thing,” I replied, “which will transport me to a better place.”

By the fourth morning, the strange feeling had gone. Instead I felt as if I’d had a double espresso before I’d had my first weak instant. As I write this, ten days in, I’m much better in the mornings, am sleeping a normal seven to eight hours and I drink less coffee. I am nicer to my family. Hard science, or placebo? Who cares? For the first time in my life, I’ve become a morning person.

For more information on Litebook, ring 08700 433842 or go to www.litebook.info For other light-box products and light-box advice see www.outsidein.co.uk