Last Night's Viewing: Supersized Earth, BBC1 Goodnight Britain, BBC1

I try to steer clear of principles as a television reviewer but I have one fixed rule that I won't break. If you're prepared to swim through toxic sewage to get my attention, the very least I feel I can do is watch your series. Well, one episode of it anyway. Last week in Supersized Earth, Dallas Campbell did just that, descending into a swill of chemicals, waste water and Mexican excrement as part of a report on Mexico City's sewage system. I'd only read a brief description of this total commitment to presenter immersion, but it was enough to trigger a dutiful viewing of episode two, in which Campbell further demonstrated his readiness to put himself on the front line.

His most stomach-churning exploit this week was to inspect the building works on the fourth Yangtze river bridge, which involved walking along the catwalk underneath the suspension cable, a hymn to Chinese safety standards constructed from chicken wire and the sort of planking you might find round the back of an allotment shed. Sheer terror temporarily disabled Campbell's usually fluent deployment of statistics: "I don't even know how high we are," he gasped, as he began to edge down a gangway of ski-jump steepness. "Stupid high."

I'm slightly at a loss to describe what Supersized Earth is about. Engineering? Social change? An opportunity to make Campbell do thrillingly ghastly things? This week, perversely, it seemed to be about how we've successfully shrunk the Earth through advances in transportation, making it far less supersized than it used to be. But it's probably best if you don't try to pull out a coherent thread and just treat it as the television equivalent of a flick through the technology section of the Guinness World Records. Last night, for example, Campbell conducted a marriage ceremony in a drive-thru wedding chapel, visited Gander airport in Newfoundland and watched a Soyuz rocket take off, all notionally in pursuit of the theme of "movement".

There were a lot of whizzy computer graphics (London's Underground system lifted above ground so that you could see the invisible net it weaves across the city) and quite a few of those statistical analogies that can leave you more baffled than a simple number. Somebody had worked out, for example, that you'd need a parking lot the size of the Grand Canyon to garage all the world's cars and we were also told that the total mileage travelled by shipping containers is enough to get to Neptune and back 145 times. How far away is Neptune? Well, just imagine how far all the world's shipping containers travel in one year. It's a 290th of that.

"We're a nation in the grip of a sleep crisis," we were told at the beginning of Goodnight Britain. Got that? A crisis. Not just some people having a bit of trouble getting off to sleep. A national crisis. Which has us in its grip. And meanwhile the Government is worrying about things like national deficits and flood defences and trivia like that. Anyway, the BBC, at least, is not sleepwalking to disaster. They've constructed a shiny Sleepmobile surveillance vehicle and staffed it with two sleep experts to check out five representatively sleep-troubled Britons.

Kathryn has parasomnia (which means she has a tendency to wander into her flatmate's room screaming, which is not exactly brilliant for the flatmate's sleep either), Paul snores and Sheila does virtually everything at night except sleep, including baking industrial quantities of cupcakes. She's welcome for a no-sleepover anytime. And, of course, insomnia is horrible and serious snoring almost as brain-shredding for an unlucky partner. But neither fact makes it any more interesting to watch people tossing and turning in the dim monochrome of a night-vision camera. If they'd shown it a bit later in the evening, around bedtime, I think they'd have gone some way to solving the crisis.

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