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Let’s twist again

Given that the average person twists and turns up to 100 times during a night’s sleep, why is it so unusual for anyone to fall out of bed? Does the human brain have a built-in warning system that is triggered when one’s body goes near or over the edge?

• Some 25 years ago, at the University of Edinburgh, Geoffrey Walsh and I investigated the reasons why adults do not usually fall out of bed while asleep. Since no one can know what movements they carry out during sleep unless some form of recording is used, we devised a simple experiment.

Volunteers slept on a very wide mattress in a warm room, with no coverlet so that they would not be able to detect in their sleep where they were in the bed. Their head position was noted from a choice of four positions: nose to left, nose up, nose to right, or nose down. The apparatus was unsophisticated, and comprised a rugby scrum cap onto which I stitched a circle of plastic tubing complete with a short piece of glass tubing. The tubing contained some mercury, and I thrust some needles through the tubing wall at suitable points and attached a dry battery so that small voltages were generated according to the head position. These were recorded all night on an electroencephalographic recorder.

Periods of sleep and waking were recorded by arranging for a small sound to be made at around 10-minute intervals. If the volunteer was awake and heard it, they pressed a bell push attached to their clothing. This allowed us to discount movements made during this time. During sleep, of course, no response was recorded.

Participants turned at irregular intervals throughout their sleep, for example, nose to left, nose up, nose to right, then back again. But they never turned nose down. As a result, they did not roll over and over so that they would fall out of bed. Instead, they remained in roughly the same position all night.

At what age does this behaviour appear? Because we could not leave young children alone with such fascinating apparatus on their heads, I simply watched my niece and nephew, aged about four and two, in their cots over a period of some six hours while they slept. During the night they did turn nose down from time to time. So they could turn over and over, and could have fallen out if their cot sides had not stopped them.

I concluded that quite early in life we learn that it is difficult to breathe if we turn nose down, and we avoid it even when asleep. As a result we won’t roll out of bed.

John Forrester

Edinburgh, UK

• If you find not falling out of bed in one’s sleep an impressive skill, spare a thought for sailors. In some ships they still sleep in hammocks; and the naval version, called a mick, is slung tight and level. Though it does seem to reduce sensitivity to the ship’s motion, any sleeper who cannot lie flat and still in a mick is a hostage to fortune. It dumps you instantly if you so much as breathe asymmetrically, and yet thousands of sailors have slept soundly in them for centuries.

Antony David

Cape Town, South Africa

• The ability of people to adapt to unfamiliar sleeping situations (from broad beds to narrow beds to hammocks to futons spread on a floor) suggests that on top of the processes investigated by Forrester and Walsh, we are somehow able to tell ourselves how much we can move before we fall asleep. In some ways this is similar to telling ourselves what time to wake up, which many people can do without an alarm clock – Ed

Plant poser

Did all the oxygen in Earth’s atmosphere come from photosynthesising plants? If not, where did it come from?

• In the Earth’s crust, oxygen combines with all the most common atoms to form water, rock, organic compounds and almost everything around us. Spontaneous free oxygen is about as likely as finding round rocks perched on steep slopes. Such rocks would imply that something had pushed them uphill more strongly than they could roll downhill.

Similarly, any free oxygen about us has been torn from its compounds with more than its bonding force. And that is a lot of force that only a few things are able to do. Ionising radiation, such as X-rays, can do it, but there is little of that about. Visible light does it laboriously, step-by-step through photosynthesis, the only process that could release the breathtaking amount (no pun intended) of oxygen that we see about us. On the back of an envelope I’ve calculated this to be perhaps 10¹⁵ tonnes.

How much oxygen plants actually produce is another matter. The chloroplasts used by plants to photosynthesise are thought to have originated as symbiotic cyanobacteria. So, in effect, all our oxygen came from photosynthesising bacteria.

Jon Richfield

Somerset West, South Africa

• Practically all of the atmospheric oxygen is of biological origin. The main culprit, however, is not plants but humble cyanobacteria. These single-cell organisms, which were present on Earth more than 3.5 billion years ago and pre-date plants, were initially responsible for all oxygen production and are still responsible for more than 60 per cent of current oxygen production.

Cyanobacteria come in many varieties and are sometimes called blue-green algae, although they are not really algae. A species of cyanobacteria present in the ocean, Prochlorococcus marinus, is both the smallest photosynthetic organism known and the most abundant of any species on the planet. It was only discovered in 1988.

Elmars Krausz

Canberra, Australia

This week’s question

Waxing lyrical

What affects the different shadings of ear wax? Sometimes mine is a light honey colour, other days it is very dark orange/brown. And why does its consistency change?

Tony Columbine

Birmingham, UK