Animals

 How Some People Can Sleep Through Anything

Do you know someone who can sleep through almost anything? Why don't they wake up?

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Surely you've heard the term "light sleeper"--they're the people who complain every morning about having been woken up by the slightest provocation. On the other hand, some of us are "heavy sleepers": they're the ones who have five different alarm clocks set all over their bedrooms because they can sleep through practically anything. It turns out that it may have nothing to do with their sleeping habits or how late they stayed up the night before, but with something called "sleep spindles". These aren't widely understood, but they emanate from the area of the brain known as the thalamus, which regulates consciousness, and alertness. They begin during the second stage of sleep, right after you fall asleep and have been connected to memory consolidation and learning, brain development and plasticity, and may even be linked to diseases like Parkinson's, Alzheimer's, and schizophrenia.

Current research suggests that sleep spindles are what decides if a person is a heavy or a light sleeper. Researchers played sounds for sleepers at increasing loudness throughout the night and they discovered sleepers with higher rates of sleep spindles were better at sleeping through the noises. Logically, this is somewhat counter-intuitive: we would assume that heavy sleepers brains are somehow less active, but this seems to show that they're actually more active. A study published in 2010 in Current Biology says the thalamus' sleep spindles essentially drown out external stimuli, causing people to appear to be heavier sleepers. Sleep studies have found women produce more than men, and as we get older the number of spindles dwindle, (which might be why some people report having a harder time staying asleep as they get older).

Where do you fall on this spectrum: Do you consider yourself a light sleeper or heavy sleeper? Let us know in the comments.

Learn More:
Sleep Spindles: Where They Come From, What They Do (NIH.gov)
"Sleep spindles are extensively studied electroencephalographic rhythms that recur periodically during non-rapid eye movement sleep and that are associated with rhythmic discharges of neurons throughout the thalamocortical system."

Reduced sleep spindle activity in schizophrenia patients (NIH.gov)
"Sleep spindles are generated by the thalamic reticular nucleus in conjunction with specific thalamic nuclei and are modulated by corticothalamic and thalamocortical connections."

Busy Brains Make for Deeper Sleep (Science Mag)
"Sound sleepers share a surprising secret: a bustling brain. A new study reports that people who can sleep through anything show more frequent bursts of brain activity called sleep spindles than do their light-sleeping counterparts."

Study: How Our Brains Make Us Light or Heavy Sleepers (Time Magazine)
"How many times do you wake during the night? Do the slightest disturbances - the sound of a toilet flushing, say, or the TV in the next room - rouse you from sleep, while your partner slumbers soundly through a thunderstorm?"