Good to sleep your brains out

THE Bard knew the value of sleep. But how does this marvellous therapy work?

There are several theories. Giulio Tononi and Chiara Cirelli, professors of psychiatry at the University of Wisconsin-Madison, put forward a new one in the current edition of Scientific American. They want to turn accepted wisdom on its head.

All animals, even flies and cockroaches, sleep. Hibernators are out for the count all winter. Swifts and house-martins cat-nap on the wing. Falling asleep is risky; a predator can creep up on you, the house may go on fire or burglars may break in. Sleep must be essential to an animal’s welfare that it’s prepared to take such a risk.

We oscillate between two sleep phases during the night. Electrodes attached to the head show distinct brain-wave patterns for each. We dream during shallow ‘rapid eye movement’ (REM) phases. The eyes are less active in deep non-REM sleep. Birds show the two phases, but they open an eye every few minutes, to check for danger. They do so less frequently during non-REM sleep. Males blink more than females and birds sleeping together open eyes less often than ones on their own. Sea mammals have a sleep problem; whereas breathing for most creatures is automatic, they have to swim to the surface to draw breath and must be conscious to do so. Whales and dolphins must keep half the brain awake while the other half sleeps.

Our brains, paradoxically, are as active during sleep as when we are awake and use almost as much energy. Billions of new neural connections are made every day. A fifth of the body’s energy is consumed by the brain, the largest demand for its size of any organ. Where does all that energy go when we are unconscious, immobile and virtually paralysed? A mysterious process is under way, but why must we be disconnected from the outside world during it? Every Leaving Cert student knows that recall is better following a good night’s sleep; spending hours in the Land of Nod seems to consolidate newly formed memories. According to the traditional view, sleep reinforces and strengthens the imprints of experiences we had when awake; the brain circuits supporting them become reactivated, strengthening recall.

Tononi and Cirelli, however, argue the opposite; they claim sleep weakens the connections between nerve cells. They point out that everything we experience, no matter how trivial, is accompanied by contacts between neurons in the brain. When we sleep, the brain activates memory circuits old and new. ‘You get a glimpse of this neural free-for-all in dreams’, they say. ‘If all of these connections were to be retained and strengthened, the system would soon become overloaded and, in any case, there would not be enough energy to drive it’. If, instead of strengthening brain connections, the opposite were to happen, the unimportant memories would fade away and only the most significant new ones would survive. Sleep, they argue, is a circuit-weakening process reducing energy demand. Their thesis is supported by studies on rodents. Chemicals which facilitate connections between nerves in rats’ brains, multiply during waking life but decline in sleep.

To accomplish this nightly task, we must be unconscious. Otherwise the brain would be bombarded with new material. Shutting down access to the external world allows the brain to renew itself without having to deal with further experiences. ‘The risk we take by becoming disconnected from the environment for hours at a time is the price we pay for this neural recalibration’ they claim.

If we are awake for too long, small chunks of the brain take quick naps. ‘How many errors of judgement, silly mistakes, irritable responses and foul moods result from (such) local sleep’, they ask.

* Perchance to Prune. Giulio Tononi & Ciara Cirelli. Scientific American, August 2013.