Am I descending mountains in my sleep?

It is caused by the atmospheric pressure change. You can change the barometer "Watch Mode" setting to the mode "Barometer" if you want to prevent it. However, then, do not forget to set it back to the mode "Altimeter" or "Auto" when you need the Altimeter functionality.

I don't have an Instinct, but on your 4 hour graph isn't the stacked numbers on the left the high and low for the past 4 hours and the number in the circle current altitude? 

tess said:

isn't the stacked numbers on the left the high and low for the past 4 hours and the number in the circle current altitude? 

Yes, of course, it is, but as I wrote the change of the elevation during the 4 past hours is caused by the atmospehric pressure change, and since the barometer is locked in the altimeter mode (and cannot calibrate itself with GPS), it considers the change to be caused by the change of elevation.

trux said:

Yes, of course, it is

I was just confirming and trying to figure out why the OP was saying:

3304570 said:

Instinct shows that it descended 100m or so in the last 4 hours

In the attached photos, the greatest difference is 18m which is a far cry from 100m and certainly not "descending mountains".

trux said:

the change of the elevation during the 4 past hours is caused by the atmospehric pressure change

Yes, I suspect this too.  As I said earlier, I'm just trying to figure out what the OP is talking about and why they think there may be a problem with the software.

trux said:

but as I wrote the change of the elevation during the 4 past hours is caused by the atmospehric pressure change, and since the barometer is locked in the altimeter mode

You're speculating because the OP never stated what mode they're using.  A minor change as the photos indicate can also happen in "auto" mode too. 

My fault.

it’s not a 100m difference, but about 20m.

Thanks for the responses everybody. I will try that setting, although I find it unintuitive. If the watch is not moving at all, why adjust the altitude?

Well, it shouldn't but if you're using auto mode the watch is basically guessing why there was a change in pressure. It works fairly well, but it's not perfect.  In altimeter mode it takes pressure changes as a change in elevation and in barometer mode it locks elevation and takes pressure changes as a change in weather patterns. 

I have mine in auto and this have never happend. 
garmin is not like casio barometer. It know if you are moving or not. It is not moving when sitting on a drawer so this should not happend

Bullet said:

It is not moving when sitting on a drawer so this should not happend

After using many Garmin devices with a barometric altimeter, I can tell you it does happen if in auto mode. As you said, it shouldn't, but I have yet to read one person who says it works perfectly in auto mode.  It works well for me the majority of the time, but not every time. 

3304570 said:

If the watch is not moving at all, why adjust the altitude?

When you are outdoors with GPS turned on, the watch can check the GPS (or DEM) elevation, and auto-calibrate. When you are indoors, it cannot, and there is no easy way it can distinguish whether you are for example slowly climbing floors, take an elevator, or stay really all the time at the same altitude and those pressure changes are caused by the atmospheric change, or whether there is a mixture of both cases.

tess said:

You're speculating because the OP never stated what mode they're using.  A minor change as the photos indicate can also happen in "auto" mode too. 

No speculation, perhaps I just insufficiently explained what I meant: The auto-mode works so that it either locks the mode into the barometric, or into the altimetric measuring, depending on conditions (for example when it detects an elevation change with the help of the GPS), and for so long the condition do not change either. There is no combined  mode. The barometer is always in one of the altimeter/barometer modes - the pressure change either counts against the elevation, or against the atmospheric pressure, never both together, even if you are in the auto-mode.

So in this specific case the barometer was locked in the baro mode, and it does not matter whether the option in Instinct Settings was set to the auto or the baro mode.