Do astronauts suffer from hypnic jerks?
June 16, 2012 10:53 AM   Subscribe

Do astronauts suffer from hypnic jerks? That sensation of falling as you're drifting off to sleep that causes your body to thrash to catch you, but you're already safe in bed. I would think that since astronauts have been constantly falling that their brain would be desensitized to it. Does anyone have some definitive answers?
posted by borkencode to Grab Bag (12 answers total) 9 users marked this as a favorite
Not a definitive scientific answer, but anecdotally, my hypnic jerks do not always involve falling, and sometimes involve a movement that in my dream is voluntary. Sometimes I jerk because I imagine tripping or jumping or something, and the jerk seems to be related to that, rather than the falling.
posted by LionIndex at 11:18 AM on June 16, 2012

I'm no expert, but astronauts don't feel like they're constantly falling (even though, from our reference frame, they are) because it's constant, not because their brains ignore the feeling of falling. Inertia is relative to your frame of reference; for them, the noticeable part is when their motion changes relative to the norm, just like if you're in a constant-speed train car you can move about as if you were on a street. You only notice the motion of the car when it changes speeds.
posted by axiom at 11:56 AM on June 16, 2012 [2 favorites]

...So my guess would be that yes, they would still suffer hypnic jerks, inasmuch as their brains aren't specially desensitized.
posted by axiom at 11:57 AM on June 16, 2012

My cousin the navy test pilot (who is in the middle of his astronaut training, wooo) says he still experiences this before falling asleep. I am currently trying to convince him to ask some actual astronauts this question.
posted by elizardbits at 12:05 PM on June 16, 2012 [4 favorites]

axiom, I don't think that's right--The forces experienced by an astronaut in orbit are basically the same as the ones experienced by a person in free-fall near the surface of the Earth. Each person is feeling the approx. 9.8 m/s^2 acceleration toward the Earth. It's just a question of whether you feel it for a minute while you are skydiving vs. feeling it for months while you're living in a space station.

"Beware losing your orientation, however. If your instincts take over, your brain starts telling you, 'You're falling, reach out and catch yourself.' Your arms and legs flail until you regain rational control and convince your brain you are okay.
posted by jjwiseman at 12:13 PM on June 16, 2012

jjwiseman -- from farther down your link:

"Weightlessness is not like endlessly falling, although in reality, that is what it is. Nor is it like a roller coaster, although being in an airplane flying the path of a roller coaster is one way to create weightlessness for about 20 seconds at a time.

"In weightlessness, you are effortlessly floating, because all of the acceleration forces on you add to zero."
posted by brainmouse at 12:16 PM on June 16, 2012

Well, first of all, astronauts are hardly homogeneous in their responses to weightlessness. Some get severe motion sickness the first hours or even days in space.

It is noted, however, that astronauts generally sleep better* (with constant monitoring from the ground as to their schedules, especially since there is no natural day/night cycle in space), and since disturbed or interrupted sleep is associated with the prevalence of hypnic jerks, that may mean their overall incidence is reduced.

* Note, this is ISS; the Shuttle was crowded and busy all the time, so astronauts slept poorly on Shuttle-only missions.
posted by dhartung at 12:21 PM on June 16, 2012 [1 favorite]

"Weightlessness is not like endlessly falling, although in reality, that is what it is."

brainmouse, it's still the case that the forces acting on you while you're falling are almost exactly the same ones acting on your while you're in orbit. they say weightlessness really is falling, but it's not "like" that, and the question is why isn't it like it? I think they're implying with the "If your instincts take over, your brain starts telling you, 'You're falling..." bit that you just get used to it. Because as far as I can see the physics is exactly the same. If you can explain why the physics is different I'd like to hear it.

Consider a vomit comet flight. During the "weightless" portion, you actually are falling. It's just that the plane is falling with you at the same time. It's like skydiving, but you happen to be surrounded by a plane at the same time.
posted by jjwiseman at 3:26 PM on June 16, 2012 [1 favorite]

The free-fall terror feeling is in the acceleration. Astronauts in weightless orbit are not accelerating, or else they would hit the earth or shoot into space. In the vomit comet, the "oh my god I'm falling" feeling is that brief moment when the plane drops out from under you. After that, you are in stasis as far as your own reference is concerned. It's not the same as falling because your only reference point, the plane, is also falling. It's the same thing as why it's not harder to walk forward in an airplane than it is to walk backward.

And I think the hypnic jerk thing is backwards: the jerk makes you think you are falling, as opposed to falling making you jerk.
posted by gjc at 4:50 PM on June 16, 2012 [1 favorite]

Astronauts in weightless orbit are not accelerating, or else they would hit the earth or shoot into space.

Nope. They're constantly accelerating towards the earth - that's how they stay in orbit.
posted by LionIndex at 5:07 PM on June 16, 2012 [2 favorites]

What people typically mean when they say, "think you're falling" is not, "think you are in the middle of a sustained process of falling" - i.e., halfway through a skydive, or weightless in space/free-fall. It's "think you're starting to fall" - you've just tripped, stepped off something, suddenly lost your balance or otherwise had a change that means you need to twitch to catch yourself. So I don't think the two would be related.
posted by Lady Li at 5:02 PM on June 17, 2012

Mod note: Final update from the OP:
An astronaut says yes!
posted by jessamyn (staff) at 7:41 AM on November 7, 2013 [6 favorites]

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