Why Spring Nights Catch You Out

April and May in the UK are the season of the smug afternoon. You pitch your tent in a t-shirt at four o'clock, the sun is on your back, and it feels as though summer has arrived early. By ten the same evening you are wearing everything you own and wondering why the sleeping bag that was perfectly adequate last August now feels like a sheet of newspaper.

Two things are working against you. The first is the size of the swing: a clear spring night with no cloud cover can lose eight to ten degrees between teatime and dawn, and frost hollows and valley bottoms run noticeably colder than the hillside above them. The second is the ground. Soil and rock hold very little heat even after a warm day, so the underside of your bag is fighting a surface that quietly pulls warmth out of you all night. Get either wrong and no amount of shivering will fix it.

Read the Season Rating Properly

Every serious bag sold in the UK carries a temperature rating, and most of us read only one number from it. The three that matter are comfort, limit and extreme.

• Comfort — the temperature at which an average adult woman sleeps comfortably. If you sleep cold, treat this as your working number.
• Limit — manageable for an average adult man curled up, but with no margin for a bad night. Fine for one night, miserable for three.
• Extreme — a survival figure, not a comfort figure. Ignore it when planning.

A bag rated "comfort 5°C" is really a summer bag. For spring camping, when a ground frost is entirely realistic in the Pennines, Snowdonia or the Scottish Borders, look for a comfort rating of 0°C to -5°C. Where a bag is labelled only "3-season", assume that means comfortable in southern England in June. Every rating also assumes you are wearing a base layer, have eaten properly, and are lying on an insulated mat. The bag is one part of a system, not the whole thing.

The Mat Matters More Than the Bag

If you change one piece of kit this spring, change the thing you lie on. A well-insulated mat will do more for a cold night than an extra 200 grams of down.

Look for an R-value — a measure of how well the mat resists heat loss. Around 1.0 to 2.0 is a summer mat; 3.5 to 4.5 is a solid three-season choice for spring. A plain inflatable airbed with no insulation inside is the worst offender: the air inside it circulates and cools you from below, which is why so many people wake at 3am convinced their bag has failed. It hasn't; the mat has.

• Add a cheap closed-cell foam mat under your inflatable one for an easy extra layer of warmth and puncture protection.
• Shake your bag out vigorously before bed so the loft recovers, and never store it compressed between trips.
• Synthetic fills cope better with damp British conditions; down is warmer for its weight but needs keeping dry.

Layer, Don't Bulk

Sleeping in your day clothes feels logical and rarely works. Damp, sweat-cooled fabric pulls heat away from you, and bulky seams crush the loft of the bag where it matters. Instead, change into a dedicated set of dry base layers kept for sleeping only, and keep a pair of dry socks and a hat in the same dry bag.

A silk or thermal liner is the cheapest warmth you can buy. A silk liner adds a couple of degrees; a fleece or thermal liner can add five to eight, and it keeps the inside of the bag clean. A hat makes an outsized difference because a surprising amount of heat leaves through your head and neck — a buff pulled up over your ears works well. If you sleep cold, a lightweight down jacket worn inside the bag is more efficient than a heavier bag you only need twice a year.

Pack a Cold-Night Kit

• A leak-proof bottle filled with hot water and wrapped in a sock, used as a hot water bottle at the foot of the bag.
• Spare dry socks and a warm hat, kept in a dry bag and used only for sleeping.
• A foam sit mat to stand on while you change, and to slide under your hips if the ground bites.
• A flask of something hot for the last half hour before sleep.
• A liner, if your bag is only just up to the forecast.
• A dry bag for the clothes you wore during the day, so damp kit stays away from your sleeping area.

Small Habits, Big Difference

Eat something substantial before bed — digestion generates heat, and a cold camper is often just an underfed one. Have a hot drink, then visit the loo before you get in: a full bladder means disturbed sleep and a cold walk at 2am. Warm yourself up first with a short walk or a few star jumps rather than climbing in cold and expecting the bag to do the work.

Once you are in, resist the urge to burrow with your face inside the bag. Your breath is full of moisture, and a damp bag loses loft fast. Keep the hood snug around your shoulders, leave a small vent if you are too warm, and if you wake cold, add a layer before you start shivering properly — it is far easier to stay warm than to get warm again.

Do those things and a frosty spring morning becomes a pleasure: a quiet, crisp start with the tent flysheet stiff with ice, and a brew in your hands while the sun climbs over the ridge.