The brain only conserves 15–25% [1] of energy during sleep and does not appear to be a reason sleep evolved. It seems that the pressure between the cerebral spinal fluid and the interstitial fluid cannot build up to the required level while the brain is awake and fully active.

[1] http://jap.physiology.org/content/70/6/2597

He's not talking about the brain conserving energy (the brain doesn't use that much energy anyway) but rather the organism as a whole by not performing other physical activity (e.g. an early human looking for food during the night when the African predators are active.)

Well, IIRC the brain receives 25% of the body's blood flow. I think that means it's using a lot of energy (but I'm not a doctor, or a physiologist, or a biologist...)

I think that's true. Here's more support for the idea that the brain actually uses quite a lot of energy:

"Although the average adult human brain weighs about 1.4 kilograms, only 2 percent of total body weight, it demands 20 percent of our resting metabolic rate (RMR)—the total amount of energy our bodies expend in one very lazy day of no activity."

http://www.scientificamerican.com/article/thinking-hard-calo...

The figure I've seen is 20%. If you burn about 2000 calories a day that works out at about 97 watts on average. 20% of that is about 19 watts. Bit more detail here:

http://heliosphan.org/bodyfatenergy.html

Which is still insanely low if you think about it's capabilities compared to the i7 sitting next to my legs, eating 100+ W right now.

Despite all of the progress in integrated circuits, the best we have is still nowhere near theoretical limits for computation per unit energy or unit space.

> "learns nothing, make no progress"

1. What makes you so sure you learn nothing during sleep? All the articles I've read about sleep's role in learning have indicated that it helps us process what we pick up during the day.

2. Why is 'progress' so important? Progress towards what? What are we chasing after?

Whilst I realise you were only questioning sleep from the point of view of its utility, for me utility alone kind of misses the point. For me the rituals around sleep are some of the best parts of daily life. I love drifting off to sleep, I love dreaming (when I'm lucky enough to remember it at least), I love waking up without any great urgency, slowly coming to my senses, wrapped in the warmth of a comfortable bed. Whilst I appreciate it wasn't your intention, I dislike the notion that our time must be spent on making ourselves better people, if we aren't causing great harm isn't enjoying what we do reason enough to do it?

RE 2. I agree with your sentiment, but I think GP didn't intend to imply we're supposed to want progress, only that when you have two competing species, the one that progresses faster will outcompete the other one. Life forms aren't fitness maximizers, they're adaptation executors - they do stuff because they like doing stuff, not because they recognize evolutionary fitness as important.

I'm not arguing against any points you made, but I would like to give a reminder that in human history it's a relatively recent luxury to be able to enjoy sleep like you are describing. It took a lot of 'progress' to get to do that! And still many people are not able to.

Do you mean to say "fail to die faster"? You can't hasten evolution by doing stuff, unless that stuff affects you passing on your genes to your children. You pass the exact same genetic materiel regardless of how many pushups you do or skills you learn.

More pushups and more skills could mean more offspring.

Funny, it struck me the opposite way: individually prone and helpless for 8 hours a day, vs being in a community where one is theoretically protected.