Why elasticated? Because the average human turns over 40 times a night.

So? Traditional bags can be too tight: any movement is a struggle.

Like a straitjacket? Wakes you up, drives you mad. These let you move more easily.

What's the trade-off? Increased weight, risk of cold spots.

So how do they do it? The UK's Mountain Equipment just elasticate the knee area, so the vital chest and feet don't lose heat. Easy to sit up and move around in the bag.

Other makes? So far, Canada's Sierra Designs. They elasticate the full length, but aren't on sale here yet. More firms will follow.

Are they more expensive? ME's part-elasticated Sleepwalker is around £100. Synthetic rather than down insulation keeps the price low.

So it's the coming thing? Retailers say they're walking out of the shops.

What else is new? Baffles. Top-of-the-range bags from Rab, Mountain Equipment and Mountain Hardware have variously-shaped baffles, which keep the goose down well spread out. Lots of "fillpower" and "loft" .

Stop showing off. Fillpower measures the height of one ounce of down - 450 is about average, but the best bags have 700-plus.

And loft? Loft means nice and puffy: it's the height off the floor when you pull it out of its stuffsack and lay it out.

How much do the top ones knock you back? Between £300 and £370. But they keep you warm at 30 degrees below.

I'm only going to Cornwall for the summer. Then pick a good all-rounder for about £100.

Where do I get one? Specialist outdoor shops and chains: Cotswold and Field and Trek. Visit also www.snowandrock.com, www.rab.uk.com, www.sierra-designs.com, www.mountain-equipment.co.uk and www.mountainhardward.com.

What's the alternative? A couple of blankets; a touch of hypothermia.