When I was a young lad I remember being tired in the middle of a long day. My dad said to go lay down for a few minutes before we had to leave and I told him I wouldn’t be able to nap in such a short period. He countered that just vegging out for a bit without falling asleep would be almost as good as a nap. Well, it turns out, while I’ve come to find many of my dad’s tales weren’t quite correct, he was actually onto something with his awake resting idea.
Giulio Tononi’s group at U. of Wisconsin published a fascinating paper back in 2011 entitled ‘Local Sleep in Awake Rats’. The title pretty much says it all: sleep-deprived rats had cortical neurons that showed signatures of being asleep while the rats were still doing awake rat things. In particular, local regions had their neurons stop firing while slow waves—which are characteristic of sleep state—increased. See the figure below for a visual explanation.
“Off” period in an awake rat. Notice the slow-wave (boxed) in the LFP while multiple neurons in the MUA (multi-unit activity) quiet their firing. Source.
This actually wasn’t a huge surprise, since we’ve known for some time that all kinds of animals sleep half a brain at a time. In fact, from an evolutionary perspective, I’m more surprised we’ve survived so long while spending 1/3 of our lives in an unconscious, assailable state.
But of course this study was only in rats, which aren’t always the best models for primates like us. In this spirit, Valentin Dragoi’s group at the University of Texas Medical School presented results at the 2013 Society for Neuroscience conference titled ‘Occurrence of local sleep in awake humans using electrocorticography’ (don’t you just love these simple, straightforward titles?). They showed sleep-deprived humans, while in a ‘drowsy’ state, were showing these symptoms as a result of local parts of their brain going to sleep just like in Tononi’s study on rats! Specifically, they found increases in slow, delta waves correlated with decreases in faster, more cognitively active theta waves the longer people were awake.
Tying this all together: ‘sleep’ isn’t a binary state. When you’re ‘half asleep’ and performing more poorly as a result, it’s literally because parts of your brain are catching some Zs. This proves something conventional wisdom already knows: you should be making decisions after a good night’s rest. Further, getting sufficient sleep is key to your daily productivity. Finally, I don’t think it’s too much of a stretch to say you don’t have to actually fall asleep to rest your brain—as my dad once suggested—but chilling out for a bit will still have some restorative power.
One question these results beg: what is so important about sleep anyway that all animals do it? I talked before in my post about why we dream how scientists were beginning to theorize that sleep is important for prophylactic cellular maintenance. Neurons are just like little machines, transmitting electricity and firing robustly enough to keep your mind and body going. And like any electricity-run machine, step number one is to turn it off before playing around with it, since you can’t change the car battery while the car’s on (and you also don’t want to get electrocuted). Stretched metaphors aside, since my last post on dreams, there’s been a major paper supporting this prophylaxis theory. Maiken Nedergaard’s lab at the U. of Rochester Medical Center published a paper in October titled ‘Sleep Drives Metabolite Clearance from the Adult Brain’. In particular, they found the brain increasingly clearing “neurotoxic waste” like β-amyloid during sleep. This particular toxin you might have heard of before as it is heavily linked to Alzheimer’s (although it does have “significant non-pathological activity”, so don’t hate the player, hate the game).
These are the kind of findings I love about science: research that can inform us on how we live our daily lives. Or at the least, point us in the direction of living a ‘better’ life. This is largely what pushed me into science, and further compelled me to move from molecular biophysics research to studying a higher-level topic like the neuroscience of memory.