By the third week of September, the sun is already setting in London before 7pm, and by the time the clocks go back on Sunday 25 October, dusk will have crept forward to around 4.30pm. Most people notice the shift only once it has already happened — the walk home suddenly in the dark, the lights on by mid-afternoon at the desk. Seasonal Affective Disorder specialists say that's the mistake: by the time the darkness is obvious enough to prompt action, the low mood it triggers has usually had a fortnight's head start.
Why November Is Already Too Late
The NHS describes Seasonal Affective Disorder as a type of depression that follows a seasonal pattern, with symptoms typically appearing in autumn and worsening through the darker winter months before lifting in spring. What's less widely understood is the mechanism behind the timing: it isn't the cold that triggers it, and it isn't Christmas stress — it's a drop in daily light exposure disrupting the body's production of melatonin and serotonin, the hormones that regulate sleep and mood. That disruption doesn't switch on overnight when the clocks change. It builds gradually as sunrise gets later and sunset gets earlier through late September and October, which means the biological process SAD sufferers are trying to get ahead of is already well underway a month before most people start thinking about it. Mind, the mental health charity, recommends starting light-based coping strategies before symptoms set in rather than after, precisely because the early stages — low energy, a struggle to get up, craving carbohydrates — are the ones people write off as ordinary September tiredness. Wait until the symptoms are unmistakable, usually some point in November, and you're not preventing the dip anymore. You're trying to climb out of one that's already taken hold, which is a considerably harder job.
What Happens to Daylight Between Now and the Clock Change
In Manchester, sunrise on 23 September is at roughly 7.05am and sunset at 7.15pm — a fairly ordinary 12-hour day. By 25 October, sunrise has slipped to around 7.35am and sunset to 5.55pm, and the clocks then push that sunset back to 4.55pm overnight. For anyone working a standard 9-to-5, that's the difference between leaving the house in daylight and coming home in it, versus doing neither. Scotland loses light faster still — Glasgow sees under seven hours of usable daylight by late December, against London's eight.
This is the four-week window worth using deliberately. Getting outside within an hour of waking, even for ten minutes, does more for the body's light-sensitive clock than the same ten minutes at lunchtime, because morning light exposure is what resets circadian rhythm most effectively. It doesn't need to be a country walk — standing in the garden with a coffee counts, as does sitting near a genuinely bright window rather than an artificially lit kitchen. The habit is worth building now, while it's still light enough at 7am to make it easy, rather than trying to start it in late October when the incentive to stay in bed is considerably stronger and the daylight to catch has mostly gone.
The Vitamin D Question the NHS Guidance Actually Says
Public Health England's standing advice, reiterated by the NHS every autumn, is that everyone in the UK should consider taking a daily supplement containing 10 micrograms of vitamin D between October and early March, because the sun isn't strong enough during those months for skin to produce enough of it. That's a nutrient guideline, not a treatment for SAD — there's a difference worth being blunt about. The evidence linking vitamin D deficiency directly to seasonal depression is weaker and more contested than the evidence for light therapy, and several trials have found no meaningful mood benefit from supplementation alone.
Even so, the practical case for starting now rather than in November is straightforward: most people's vitamin D levels are at their highest in September, after a summer of casual sun exposure, and lowest by February or March. A supplement bought and started in late September, from Boots, Holland & Barrett or a supermarket own-brand — all sell the standard 10-microgram dose for under £5 for a three-month supply — closes that gap before it opens rather than after. Take it with a meal that contains some fat, since vitamin D is fat-soluble and absorbs poorly on an empty stomach.
Light Boxes: What the Evidence Actually Shows
It works for most people, but not for everyone.
Light therapy is the intervention with the strongest evidence base for SAD specifically, and it's also the one people leave latest, usually because a light box feels like an admission that things have already got bad. The Seasonal Affective Disorder Association (SADA) and the Royal College of Psychiatrists both point to daily sessions of 20–30 minutes in front of a box delivering around 10,000 lux, used first thing in the morning, as the format with the best trial support — comparable in some studies to antidepressant medication for mild-to-moderate seasonal depression, though with a faster onset of effect and none of the same side-effect profile. Lumie, the Cambridge-founded company that more or less created the UK consumer light-therapy market in the 1990s, remains the most recognised name on the market, with boxes running from around £40 for a basic desk lamp to £130-plus for the higher-output Brazil or Vitamin L models used in some NHS-affiliated trials. Buying one in late September, while stock is plentiful and before the seasonal surge in demand hits in November, is the more sensible move — and starting sessions before symptoms appear, rather than once they have, is what gives the light box the best chance of blunting the dip rather than just treating it after the fact.
Not every SAD sufferer responds to light therapy, and that's worth saying plainly rather than glossing over. A meaningful minority find no improvement, or find the daily 20-minute commitment itself becomes one more thing to fail at on a low-energy morning — in which case a combination of medication, cognitive behavioural therapy and the daylight habits below tends to do more than the box alone.
When to See Your GP — and Why the Timing of a Referral Matters
GPs can refer patients with more severe SAD symptoms to NHS Talking Therapies, formerly IAPT, for CBT specifically adapted for seasonal depression, or in some areas directly to light-therapy loan schemes. The detail that catches people out every year is the waiting list: NHS Talking Therapies services across England reported average waits of several weeks between referral and a first assessment through 2025, and longer again before treatment proper begins, according to NHS Digital's own published waiting-time statistics. Refer yourself — self-referral is available in most areas without seeing a GP first, via the NHS website's Talking Therapies finder — in late September, and treatment has a realistic chance of starting close to when symptoms are actually at their worst, sometime in December or January. Wait until you're struggling in November to make the call, and the appointment may not land until February, by which point the worst of the season is already behind you and the referral has done little more than confirm what you already knew. Anyone with a previous SAD diagnosis, or a strong suspicion based on a pattern of past autumns, should book that GP appointment or self-referral this week rather than waiting to see how the season goes. It costs nothing to be assessed early and be told you're managing fine; it costs a genuine six to eight weeks to be assessed late and find out you're not.
Building the Routine While It's Still Easy
The single most effective thing most SAD specialists recommend is also the least dramatic: a fixed wake time, held steady through the clock change rather than adjusted around it, paired with morning light exposure — natural where possible, boxed where it isn't. Starting that routine in September, while the mornings are still bright enough that it barely feels like effort, means it's already established as habit by the time late October makes it genuinely hard. Trying to build the same routine from scratch in November, in the dark, at the exact point energy and motivation are lowest, is where most good intentions about "sorting out winter" quietly collapse.
Exercise carries a similar logic. A brisk 20-minute outdoor walk at lunchtime, kept up through September and October, does more for mood regulation than the same walk taken up as a January resolution once the low has already set in — and it's considerably easier to start while the weather still allows for it without full waterproofs. None of this replaces medical treatment for anyone with moderate-to-severe symptoms. But for the far larger number of people whose autumns follow a milder, predictable dip, the four weeks between now and the clock change are the actual window that determines how this winter goes — not the four weeks after it.