Trying to understand the neurobiology of sleep and its functional purpose kept me up for about a week in the great insomnia bout of May’17 and has confounded scientists for decades. Partly because we have been trying to see the universe through a straw, but mostly because we cannot (ethically speaking) run the necessary experiments on humans that would offer the greatest insights.
Fortunately there are a few things we know about the sleeping brain, and two studies published earlier this year have gone a long way to furthering our limited understanding of the squishy thing between our ears. But before I get into the nitty-gritty science of this progress, let me explain what we know so far…
Some 50 years back, scientists began exploring the function of sleep and became particularly interested in the phase of sleep known as REM (rapid eye movement). To test the effect REM deprivation has on brain functioning, a man of questionable moral character ran a series of experiments on domestic cats. He placed the cats on a small flower pot surrounded by cold water, so small that when the cat would enter REM sleep and relax it would fall into the water and wake up. Their physiological and behavioural responses were monitored for 70 days, but the cats became ever more disturbed and on average died half way through. So we learnt three things from this; REM sleep is probably important, cats really don’t like water, and they funded practically anything in the 60’s (honestly, it’s not the worst thing that went on in university basements).
Throughout the 60’s and 70’ there was a wealth of research exploring the function of REM and Non-REM sleep, and the link to memory. The theory being that sleep, and indeed dreaming, help consolidate the memories we form whilst we are awake. If you have ever watched a baby or a dog sleep you will know what this looks like as they relive the mischief they unleashed in their waking moments. In fact, some more strange cat experiments (available on youtube) show that REM sleep is what allows the body to go into a state of muscle atonia (paralysis) so you don’t act out all your dreams. However it was not until 1994 that a seminal paper was published, showing that patterns of network activation in the waking brain are ‘replayed’ during slow-wave sleep., not REM
Fast-forward to the early 00’s’s, and the field saw a new lease of life as Giulio Tononi and Chiara Cirelli came up with a rather controversial theory. They suggested that synapses (the connections in the brain that make all the magic happen) grow so vivaciously while we are awake that they become saturated. This makes our brain circuitry “noisy” and if it reached a point of absolute saturation we would no longer be able to encode any new information. So to restore balance to our personal universe, these synapses scale back down during sleep; also known as the Synapse Homeostasis Hypothesis (SHY). In other words, sleep is the sacrifice all animals make for the ability to learn.
A few months ago those two experiments I mentioned found support for this theory. Luisa de Vivo, the lead scientist working in the Cirelli lab, led a painstaking survey of brain tissue from sleeping and waking mice using electron microscopy. She and her colleagues determined the size and shape of almost 7,000 synapses over 4 years and found that synapses in the brains of sleeping mice were 18-20% smaller than in waking ones. The second study led by Graham H. Diering from John Hopkins University set out to explore the molecular changes that occur by studying the proteins in mouse brains during and after sleep. As would be expected if the synapses were shrinking, they saw that surface receptor proteins declined during sleep.
What was really interesting though is that the relative strength of synapses remained unchanged and in fact, the strongest connections remained mostly unaffected by this midnight pruning. At this point we can only speculate why these connections are preserved but it seems logical that the brain would safeguard connections that are the most useful as they have been strengthened over time. This means the types of connections the brain reinforces day after day; red light = stop, loud bang = danger, tequila = fun (the important stuff), become ‘hard-wired’ whilst we sleep.
So if you want to retain what you learn in the day, be it a new language, your 756 passwords, or the lyrics to the full version of Fresh Prince of Bel-Air, the best advice is plenty of sleep and plenty of practice. Alas, there is no neuro-hack to a good memory and it turns out Mr. Michaels from year 5 was right all along, use it or lose it.
P.S The neurobiology of sleeping and dreaming is incredibly complex and I have not begun to touch upon the function of dreaming vs dream-less sleep, and lucid dreaming…so stay posted.