I am wondering how it would feel if gravity suddenly got reduced to be around 90% of what it was. Like if a person was not paying attention or was asleep, would they notice?
If it happened on the whole volume of the planet? Instantly.
I am wondering how it would feel if gravity suddenly got reduced to be around 90% of what it was.
This already happens - the acceleration on your body decreases suddenly - when you fall or stumble. Your inner ear and proprioceptors would immediately decode this as "I am falling, or there is an earthquake". A very old, very high priority reflex set deep in your hindbrain would probably wake you at once and send you running.
And as @MikeSerfas noted, a 10% gravity reductions means a 10% reduction in atmospheric pressure; nothing life-threatening, but it would be like moving instantly from sea level to a small thousand-meter (3,000 feet) mountain. You'd immediately feel a sharp pain in both ears and need to swallow to equalize the pressure; that would surely wake you up.
(Not to mention that enclosed environments like houses and offices would experience an overpressure on windows to the tune of one ton per square meter as the 1-atm air inside tries to equalize the 0.9-atm air outside. If the windows didn't crash open, they'd probably shatter).
If neither the fall nor the ear pain did it, you'd feel it for sure some seconds later, when The Earthquake happened. Because the whole Earth crust exists in a compressed state, due to the weight of the different materials it's made of. Imagine a lot of weights over different springs, slowly set after millions of years. Then suddenly one tenth of those weights disappear. What do the springs do? They decompress, and not all at the same time. This is called orthostatic rebound and, with those numbers, it would be absolutely catastrophic.
If the crust under the seas rebounds first, the sea level would rise abruptly, what is called a megathrust earthquake. Colossal tsunamis would devastate the coasts - all the coasts. A megathrust of an estimated 2 meters over a 400x100 area was at the root of the 2004 tsunami. "Several meters" gave birth to the Tohoku quake, the one that hit Fukushima. Depending on the source (and how much depth we consider), decompression of basaltic seafloor would give anywhere from twenty to one hundred meter rebound (probably not instantaneous, that much is true, but still...).
Then, the lithosphere megathrust would very likely be enough to trigger most faults in the whole world. It would also crack the domes of most supervolcanoes. In short, everyone dies.
For all this not to happen, gravity ought to only affect the ecosphere (following the geodetic contour), or the change would need to be gently spread over hundreds of years (tens of thousands would be better). Even so, the "creaking" of the Earth (volcanoes, earthquakes, tsunamis) would be very noticeable, albeit survivable.
(This whole matter came up about fifteen years ago with Robert J. Sawyer's End of an Era)
10% reduction is quite a lot.
As you say the reduction is suddenly, anyone would notice, even when asleep. It's because of the inner ear where balance is 'measured'. The reduction will feel like you're suddenly dropping a bit. The feeling of falling can jolt you awake easily. Compare it to a platform (elevator or otherwise) that suddenly shifts a tiny bit down. It instantly gets your attention.
It is the body having a warning signal, as ut certainly wants to know when you might unexpectedly fall down.
If the person is a heavy sleeper and unaffected it might take longer. It is easy to miss-attribute feelings of being lighter to light-headedness or possibly feeling incredibly energetic with every step. Yet even without scales and such we can start to notice things. Water in the shower or from the tap not falling or draining as fast. Maybe playing with keys and notice it handles differently. There will be a great amount of variability in the person and their mood at the time when they notice. Even if they notice, it might be a while before they can tell/believe it's a reduction of gravity.
Yet as your question specifies that they should notice and not necessarily know what is happening, I say very quickly.
As @LSerni pointed out, there'd be some immediate effects. If we somehow manage to survive these, people would eventually notice the change of climate. And probably die.
A 10 % decrease of the force of gravity would send Earth away quite noticeably further out in the solar system, 10 % further out, as a first order approximation. The amount of sunlight that reaches a planet decays quadratically with distance, so we should expect 10-20 % less energy from the sun. Even though Earth would remain habitable, the climate would change drastically, harvests would fail and other noticeable stuff would happen.
Precisely how bad it is will depend on what season it is when It happens. If it's in the northern winter, the part of the orbit that's furthest away would occur in northern summer and vice versa.
With all the geopolitical stuff that will follow, Earth will most likely be quite a bit less hospitable. Food will run low, especially after a year or two, and my bet is quite some people would die.
Edit in response to comments: For some reason, be it a happy accident or some deep underlying principle in physics, inertial mass, active gravitational mass and passive gravitational mass are all proportional to each other. Further, the two gravitational notions of mass seem to be the same. This is why a decrease of Earth's mass wouldn't affect it's orbit, since the decreased pull from the Sun would be matched exactly by the decreased inertia.
Without any gravitational forces, the Earth would move in a straight line and conserve its momentum. With gravitational forces, the Earth will constantly accelerate towards the sun, causing its path to bend. Currently, we sit in an equilibrium where this turning adds up just right to result in a nearly circular orbit, but other planets have orbits that are more eccentric.
If the strength of gravity is suddenly adjusted, the inertial and gravitational measures of mass would still be proportional, but the proportion would be different. That's what changing the constant means! The earth would have all its momentum, but would not be turned inwards as much. The arch would be less curved, and we would end up further away from the Sun.
Now, planetary orbits are elliptical, and as this result doesn't depend on the actual value of the gravitational constant they would remain so in our scenario. We start out moving at a straight angle from the Sun, so we must thus be at either the aphelion or the perihelion of our orbit. With more inertia than a planet should have to be stable at this distance, 10 % too much, it turns out we are at the perihelion.
It will take a little over half a year to reach our new aphelion, the furthest point of our new orbit.
It might be even worse than I originally said though, as my napkin calculation assumed a new stationary orbit. Intuitively, the new elliptical orbit will spend roughly half the time inside the 10% larger orbit, and the other half outside it. Objects in elliptical orbits spend more time being further away than being near, so we would get orbital winters that are quite bad.
I'm no physicist, and I can't give you the precise formulas for how bad it would become, but if the small eccentricities of our current orbit are enough to send us into periodic ice ages, this scenario seems pretty apocalyptic.
I maintain my position that people would notice.
(NOTE: below answer the effects when "g" is changing.. When "G" would change, there will be large scale repercussions, like e.g. the moon running out of orbit, or a planetary ice age, sun and moon will become less heavy as well, making the change come from both sides)
Science and medicine will notice in 10 milliseconds and get confused within seconds
Of course, scientists immediately realized what happened. Scales show wrong results now. Issue is, you can't calibrate your instruments properly. The kilogram calibration weight will have become 10% lighter. The pharmaceutics industry will take a hit on day one, most medicine will become decertified immediately and production will stop, until everything is corrected and certain decisions are made: suppose we apply more substance to reach a prescribed weight, could that result in overdose ?
In the comments below, Trioxidane made a valid point, which would complicate things further. Some scales may not be affected.
Industry will panic in a few seconds
Counterweights compensating for engine-driven equipment will be less effective. This happens while the equipment runs. It will cause misalignment, out of control behaviour and instability in some systems. There will be explosions, as a result of feeding excess energy into processes, because weight reports less.. Result is a wrong decision: the weight scale does not show the target weight, so keep pumping ! Suppose the container cannot reach the target weight anyhow, the tank will explode.
Satellite communications and internet affected within minutes
For media and internet, this weight change thing is a big disaster. Thousands of satellites will require orbit correction, all at the same time. Collisions happen. Many satellites position will get lost and for many commercial satellites the procedure may be too costly, or too late.. These channels will get lost permanently.
For that reason, worldwide it will be noticed, by any journalist dependent on satellite communication. Internet will be affected as well.
GrumpyYoungMan's comment: other satellites, like GPS will have to be repositioned as well. Ships can get lost. Passenger aircraft flying that night, dependent on GPS (and ILS) will run a risk to loose course and run out of fuel. Ground vehicles depending on GPS will cease to work. These activities will be grounded for weeks, or months, depending on how fast GPS services can be restored.
People will notice when they move.
For reasons other answers have elaborated on.. Doctors helping patients with revalidation will notice a sudden improvement. The high jump and far jump as well as pole jump world record will probably be broken, within a day.
Weight watcher's misunderstandings
The instant BMI improvement will be appreciated by many weight-watcher followers. Issue is, your looks won't change, despite loosing 12 kG.. the physics of this may be too difficult to explain to people, resulting in urban myth. The government did this to let people work harder. At weight-watchers, the experts should have a good story. Chance is, some obese people's condition will worsen in the long term, because obese people will "allow themselves" to fill the gap, to reach the old weight target again.
Gold price will get bzirk
What if this happens again next week ? Many gold owners will feel betrayed when the event happens. Selling their gold, taking the losses. It was supposed to be safe, it is not. Within a week, this price dip will be corrected. Reason.. an ounce of gold now contains more gold. Lots of people like to speculate and buy. Price rises again. And it will go up some more, I'm afraid.. The irrational sentiment about this now spreads rapidly, in parliament there are whodunnit questions now. Who did this.. and who are the stakeholders..
Food price will go up
Consumer products like food now require more food to reach the same weight. You buy vegetables per kG. This extra cost will be passed on to the consumer, prices will rise. This counts for any goods that are weighed and priced in kilograms. You need more now. Ships that could take xx tonnes, now take xx-10% tonnes, because there is no room on the ship to store the cargo. This will cause supply chain issues, software problems, and like in other industries, cause huge costs to solve it.
The kilogram could be reconsidered
Chance is, there will be pressure from many stakeholders, to change the standard kilogram. This will cause even more conspiracy theories. Prices have gone up, will we get 10% less now ? Consumers are starting to worry. And some are demonstrating in the streets already. This change is going to be a mess.. and cause an economic recession.
The more athletic people would notice it sooner, the less athletic later. Also people professionally dealing with masses would notice it pretty immediately.
Thanks to a diet I have lost 10% of my weight: I have gone from puffing and panting when climbing the stairs to the third floor to doing it with no struggles. I would notice if all of a sudden and without any diet that would be the case.
Imagine also whoever plays basketball or volleyball, they would have also immediately noticed if they could jump 10% higher all of a sudden.
And if who has a shop selling anything requiring to be weighted with a scale converting weight into mass, would notice a reduction in 10% of the weight in all the packed items with an indicated weight.
If you want to learn about a relatable sensation when experiencing reduced gravity, go play on a seesaw. As you go up and down there are moments when you experience a bit more and a bit less gravity. You feel a bit less when you are getting to the top. A similar feeling happens on a boat (depending on how agitated the sea is), in a plane when it is reducing altitude, or in a fast elevator when it is going up and about to stop.
Our inner ear is naturally calibrated to 1G. Getting a constant 0.9G might feel like being in an airplane that is slowly going down until you readjust. I ised to travel a lot, I can tell you many people sleep through it. I imagine frequent travellers may even ignore it.
That's the individual experience of 0.9G in the human body in a safe, controlled environment. If that 0.9G is global, though, you get the end times as some users have already pointed out. I specially like Serni's answer and I encourage you to read it whole and in detail. My TL;DR version of that answer, and which is what I believe would happen is: you relieve the crust of a planetary amount of pressure, the planet puffs as the crrust pops up. Oh the humanity...