Is This Why We Sleep?

Why do we sleep? Evolution works is a fussy, erratic, back and forth way, taking information systems from one type of project and then applying it to another – and another. As animals moved from sea to land, fins become hands; when they returned to the sea, the hands began yet different fins.
But sleep has been confusing people for a very long. What is the point of going “offline” for large parts of your life when you could be mucking about awake all the time?
It turns out you rest for learning and memory – in surprising ways. Interrupt sleep and you don’t learn.
Even when the issue is sex.
Recent studies at Washington University and University of Wisconsin highlight two important results: 1. There’s a sleep gate in fruit flies that’s small and manipulable 2. To learn you need to sleep. (A summary of some of the results is available in a fine article by Ed Yong, http://blogs.discovermagazine.com/notrocketscience/2011/06/26/to-discover-the-point-of-sleep-scientists-breed-flies-that-nod-off-on-demand/). Another discussion of the same data was made by Dr. Michael Breus (http://www.huffingtonpost.com/dr-michael-j-breus/sleep-long-term-memory_b_886998.html)

Sex, Learning, and Sleep

First, Brian Donley of Paul Shaw’s group at Washington University discovered a group of perhaps 20 brain cells in fruit flies that turn on and off sleep. By manipulating sodium ion channels to become sensitive to temperature controls, Donley and colleagues could turn fly sleep on and off just by turning up the heat.
Then they did a nasty trick. They made some male flies smell like female flies. Not quite transvestism, but the male flies suddenly were very, very attractive to other males.
And they did not like that fact one bit. Male flies, even female smelling ones, wanted to be treated as males.
So when other males came round trying to mate with the false female smelling males, they were rebuffed – again and again – often with prejudice. Yet if the normal flies were next placed in an “information rich” environment surrounded by many other flies, they promptly forgot all about it. So when they rounded on the fake males, they tried mating with them again. And again.
Only one thing would prevent the naïve flies from trying to mate with the false females: Sleep.
After even a brief sleep, the flies remembered that males could smell like females and reject their advances – and they finally stayed away.

When There’s Too Much Information
It’s not too hard for researchers to overload the brain of a fly. Put flies in environment where they don’t see and can’t do much, and they won’t make many new brain synapses.
Surround them with lots of other flies and give them an environment to move in and they make far more new synapses – increasing the number with each increase of the environment’s “richness.”
Still, there comes a point when they can’t learn anymore. They reach the maximum amount of synapses they can produce.
So what happens when they have a chance to make more brain connections? They sleep more. That’s what Daniel Bushey and colleagues, including Chiara Cirelli and Giulio Tononi at the University of Wisconsin, have been discovering over the years.
More learning in the day – more sleep at night.
And what happens during sleep? The synapses strengthen – and often decrease in number.
Synaptic pruning is one hypothesis Tononi and Cirelli have become famous for: that what happens in sleep is not an increase in brain connections, but a decrease – as the brain summarizes information and then puts it into usable form.
And because the storage space is not infinite, lots of it has to get dumped.
Not everyone agrees with Tononi and Cirelli, but support has been growing. It’s apparent that the information coming into almost any animal is endless – and no animal can take it all in.
So it needs a mechanism to recognize what information is useful and which isn’t – and ways to store that information.
Regardless of how that information processing is accomplished, much of it is happening in sleep.

Bottom Line

Sleep appears necessary for “higher” organisms to learn. The data have been most persuasively shown in fruit flies and other animals, but the human data is looking very similar – as Matthew Walker and colleagues are demonstrating at UC Berkeley.
Now the fact that sleep helps immunity; lets us avoid colds and severe infections; decrease heart disease risk; is critical to controlling weight; improves mood; helps your skin’s appearance – all this should not take away from the fact that sleep appears necessary for memory and learning.
After all, rest is regeneration. You’ve got to process and remember all that information somehow – if you want to live, let alone long and well.
And how might you remember such a lesson?
You might take a nice, restful sleep tonight.
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