For his November trip to London, Dr Norman Rosenthal has packed a very important piece of bedside equipment with his shaving kit and toothbrush. It is the size of a paperback, but it is not a book or even an alarm clock — it’s a portable lightbox.

“I don’t go anywhere without it,” says the man who learned the hard way that he needed a daily turbo-infusion of wattage to get him through the winter. For Rosenthal is the man who discovered that diminishing daylight brings on SAD — the seasonal affective disorder which translates to one in five of us as winter blues.

“If I had not grown up in South Africa with wonderful sunny weather, I might have suspected the connection between daylight and mood several years earlier,” admits the scientist whose family emigrated from Lithuania before he was born.

Had they not, he might have suffered annual lethargy and depression in childhood. As it was, he was already a medical student living in New York when he first encountered the short days of a miserable winter.

“When I arrived in the long, beautiful days of summer I was exuberant,” he recalls. “But then with autumn came extended hours of darkness, and I didn’t know what had hit me.

“That’s when I understood the stoicism of northern peoples — the need to just soldier on.”

As he moved on to the National Institute of Mental Health near Washington DC to study mood disorders, Rosenthal self-medicated, not with anti-depressants, but a crude dose of intensive light from an adapted ceiling fixture.

“It had just been discovered that bright light could alter the secretion of melatonin which we all produce overnight, but no-one was acknowledging seasonal rhythms or what it meant to suppress that secretion,” he explains.

“We all knew light enabled us to see, but as recently as 1980 it was not appreciated that light also had important non-visual effects as well.”

Using himself and an even more severely affected patient as guinea-pigs, Rosenthal produced a lifting of depression within three days by exposure to light. “Then I had to mount a bigger study in order to demonstrate it was not a one-shot wonder.”

To his surprise, a local article about his discovery produced a stream of subjects from all over the nation reporting severe winter depression accompanied by low energy, difficulty waking and withdrawal from friends and family. Women, it turned out, were three times as badly affected as men.

In 1984, Rosenthal published his landmark paper, and light therapy has been a growing business for more than 20 years. At first, patients were recommended to sit in front of great chunky light boxes for a fixed period of time, “but now there are much more sophisticated products on the market”.

Not an endorser of any particular brand, Rosenthal nevertheless believes special SAD appliances are a vital aid. “More crucial than whether they mimic full spectrum daylight is the intensity of the bulbs so that you do get enough light.”

Personally, he likes to keep a lightbox at his breakfast table, but agrees with claims that so long as a suitable lamp — undeniably a more aesthetic solution — is within your field of vision, it will do the job, though you may need to stay exposed for a longer time.

If anything surprises him, it’s the fact that SAD and its treatment are still not 100 per cent recognised by the medical community. “Many doctors see patients with symptoms of SAD but attribute them to a cold or getting over flu. Physicians are still educated by the pharmaceutical industry, and light is not a patentable commodity.”

Even in Sweden, he points out, where suicides rise sharply during the long winter, light therapy is not regarded as a legitimate treatment.

“This is astonishing, because such a high proportion of the population is susceptible.” His ongoing research has revealed that Zyban, the drug prescribed to help wean smokers off nicotine, can also be helpful in treating the symptoms of SAD.

Top tips for SAD sufferers

● Consider a dawn simulator, which mimics a rising sun to wake you with gradually brightening “daylight”. Simulators are available from www.lumie.com
● Get out from under the duvet, however much you don’t feel like it.
● Breakfast within range of a SAD lamp or lightbox for a longer infusion of intensive light.
● Get outside as soon as possible; exercise will help lift your mood.
● Take Vitamin D supplements; northerners rarely get enough from daylight in winter.
● Take winter sunshine breaks.
● Pack a portable lightbox or visor if travelling to other northern climes.
● Consider asking your GP for anti-depressants or Zyban.