Quick Answer: Swap when your bedroom’s overnight low sits consistently below about 18C (65F) for a week - not when the calendar turns. In most temperate regions that’s late September to mid-October, well after the first cool day. The better approach for the transition period is layering: keep the summer duvet and add a blanket you can kick off, rather than making one binary switch.

There’s a specific late-summer failure mode where the first genuinely cool night arrives, everyone digs out the winter duvet, and then it’s 26C for the following fortnight and nobody sleeps.

Bedding is one of the few sleep variables entirely within your control, and getting the timing right is worth more than most of what people buy to fix their sleep.

What temperature should a bedroom actually be?

There’s a fairly narrow band that suits most people, and it’s cooler than most bedrooms are.

Roughly 16 to 19C (60 to 67F) is the range generally recommended for sleep, and there’s a physiological reason for it. Core body temperature has to drop by about 1C for sleep to initiate and stay initiated - it’s part of the signal that triggers sleep onset. A warm room slows that drop, and a warm bed slows it even in a cool room.

This is why overheating wakes you at 3am specifically. Core temperature reaches its daily minimum in the early hours. If your bedding is too warm, that’s the point where the mismatch peaks and you surface out of sleep - often without knowing why, and often blaming stress or the neighbour’s car.

The bed microclimate matters more than the room. The temperature under the duvet is what your body actually experiences, and a 20C room with a 4.5 tog duvet can produce a cooler bed than a 17C room with a 13.5 tog one. This is the whole reason the swap timing is worth thinking about at all.

The single most useful thing you can buy is a cheap thermometer for the bedroom that records the overnight minimum. It’s a few pounds and it turns the entire question from guesswork into a number.

What tog should you actually use, and when?

Tog is a measure of thermal insulation, not weight or quality. Here’s the practical mapping.

Summer: 4.5 tog or below. For a warm bedroom, many people do better with just a sheet and a light blanket, and no duvet at all.

Spring and autumn: 7.5 to 10.5 tog. This is the range that covers the transition, and it’s the range most people don’t own - the classic setup is a 4.5 and a 13.5 with a large uncomfortable gap in between.

Winter: 12 to 13.5 tog. For a heated bedroom, 10.5 is often enough all winter, and plenty of people who own a 13.5 have never actually needed it.

The all-seasons duvet is the genuinely good solution here. It’s a 4.5 and a 9 tog that press-stud together to make 13.5. You get three configurations from one purchase, and the middle one is the transitional weight almost nobody owns.

A note on fill: down and feather regulate humidity better than synthetic, which is why the same tog in down often sleeps cooler in warm conditions despite the identical rating. If you overheat and sleep hot, fill type is worth as much attention as tog number.

And on toppers: a memory foam topper measurably increases how hot the bed sleeps, because the foam conforms and reduces air circulation around the body. If you added one last winter and have been hot ever since, that’s very likely the cause rather than the duvet.

How do you time the swap without getting it wrong?

Watch the overnight low, not the daytime high. The daytime high is what you notice; the overnight low is what you sleep in.

The trigger to use: overnight bedroom lows sitting below about 18C for roughly a week. One cold night is not a season change - late August and early September routinely produce a single cool night followed by a fortnight of warmth.

A three-stage transition rather than a swap:

Stage 1 - late summer (now, roughly). Summer duvet or sheet, with a folded blanket at the foot of the bed. The blanket is the adjustment mechanism - pull it up at 3am on a cool night, ignore it otherwise. This alone covers most of the transition period.

Stage 2 - early autumn. Mid-weight duvet, around 9 to 10.5 tog, or the two layers of an all-seasons set used separately. Blanket still available.

Stage 3 - when overnight lows are consistently below about 12C. Full winter weight.

Two other things to change at the same time as stage 2:

• Switch to a heavier or brushed cotton sheet. Sheet fabric affects perceived warmth substantially and costs nothing to change.
• Wash and properly store the summer set rather than stuffing it in a cupboard. Storing bedding unwashed sets body oils into the fibres over months.

If you share a bed with someone who runs hotter or colder - and most couples do - the Scandinavian arrangement of two single duvets on a double bed genuinely solves it, and it removes the timing argument entirely.

Myth vs Fact: Seasonal Bedding

Myth: A heavier duvet is always warmer. Fact: Weight and warmth are different properties. A high-quality down duvet can be very light and very warm, while a heavy synthetic one can be relatively cool. Tog measures the insulation; weight measures the fill. Buying by heft is why so many people end up with a duvet that’s both heavy and not particularly warm.

Myth: You should swap on 1 September. Fact: In most temperate regions, early September nights are still comfortably warm. Swapping by date rather than by overnight temperature is the single most common cause of a bad fortnight’s sleep in autumn.

Myth: Cotton always sleeps cooler than anything else. Fact: Cotton is breathable but holds moisture, so it can feel clammy for hot sleepers. Linen and Tencel both wick more effectively and are often better for people who overheat, particularly during the humid end of summer.

Myth: An electric blanket makes the duvet swap unnecessary. Fact: An electric blanket warms the bed before you get in, which is genuinely useful. But leaving it on overnight works directly against the core temperature drop sleep requires. Use it to preheat, then switch it off.

Myth: Weighted blankets are too hot for anything but deep winter. Fact: Depends entirely on construction. Glass-bead weighted blankets with a breathable cover are usable well into mild weather, and many people use one year-round in place of, rather than in addition to, a duvet.

FAQ: Bedding Swap Questions, Answered

How do I store the summer duvet properly?

Wash it, dry it completely - genuinely completely, since residual damp in a thick duvet causes mildew - and store it in a breathable cotton bag rather than vacuum-compressed plastic. Long-term vacuum compression damages down loft permanently.

Should I swap pillows seasonally too?

Most people don’t need to, but if you sleep hot, pillows matter more than expected because your head loses a large share of body heat. A latex or wool-filled pillow sleeps cooler than memory foam, which insulates significantly.

Is it normal to need different bedding than my partner?

Entirely normal - thermal comfort varies substantially between individuals, and it commonly differs through the menstrual cycle and through perimenopause. Two separate duvets is the practical answer rather than a compromise weight that suits neither person.

How often should the duvet itself be replaced?

Roughly every five years for synthetic, and ten or more for good down that’s been washed and aired periodically. A duvet that no longer lofts up when shaken has lost most of its insulating capacity regardless of its printed tog rating.

TL;DR:

• Swap when overnight bedroom lows sit below about 18C for a week, not on a calendar date
• Bedrooms sleep best at roughly 16-19C, because core temperature must drop for sleep to hold
• The missing piece in most homes is a mid-weight 9-10.5 tog duvet for the transition
• Layer instead of swapping: summer duvet plus a blanket you can kick off
• Fill type and mattress toppers affect bed temperature as much as tog does
• Wash summer bedding before storing it, and never vacuum-compress down

The first cool night is not the signal. A week of cool nights is, and a folded blanket at the foot of the bed covers everything in between.