PC starts to restart (no bsod) during night only

So my PC only starts restarting by itself during night around 6pm or 7pm onwards. We already tested the RAM and checked CPU and Motherboard temp, replace the thermal paste so figured overheating is not the case. What's weird is it only starts restarting at night so we couldn't figure out the problem. Is this a power shortage issue? Or something else? Thanks.

I'm using Windows 10
Ryzen 5 5600g
MSI B450M-A Pro Max II
Corsair cv550 PSU
TFORCE VulcanZ 2x8gb 3200mhz RAM

Logitech G102 Mouse
RK87 Keyboard
Acer Nitro VG-0 Monitor

Are you meaning 6am or 7am? (although neither of the two options is very clearly "during night" specially 1 month off an equinoxe). 6pm-7pm wouldn't be too off the day's maximum temp time specially in summer, did you review the temps for it? (and 6am-7am isn't off the day's minimum temp time in any season)

Are you meaning continuous restarting?

Accumulated heat? Which case and case fans do you have? Your system could perfectly be under 100W real consumption even at load with its 65W processor (unless it has a discrete gfx card), but passively cooled things (most motherboard chips, HDD/SSD, most PCI(e) cards,...) always need case cooling. Usually an exhaust case fan suffices.

If all the above is covered, I'd also look for a sw cause.

I don't leave it overnight, I just stop using it around 7pm because of the restarts. The random restarts was already there the first time I built the PC but it was only because of the wires of the Fan that was not properly plugged. Now, it only keeps restarting around the evening. I usually wake up at 9am or 10am and use it onwards until the random restarts starts to occur.

I already did check event viewer but nothing was there that tells the reason why the PC restarts or did I just miss something? What Event ID should I look for?

Nothing on Scheduled Task and Reliability History too.

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It's 6pm or 7pm onwards, evening. When I check the BIOS after the restart, CPU and Motherboard are at normal. I also observe GPU and it's normal.

Yes it becames continuous around evening. It will restart suddenly then when I open it, either the display won't bite or it would let me login to windows back then would restart again once I run an app or something. Sometimes, It would restart once I entered my pin/password.

I only use 4 Inplay Fan Case and an Inplay Case.

We already tested not using the PC at daytime (or maybe use it for like 1-2hours only) and it would still start restarting when evening comes ://

See Wake Source in Windows 10

Also see, Related Tutorials.

Maybe it has to do with the mains power quality. Does it happen to other computers in the zone? Do you notice light flickers?

I use (cheapish) surge protectors myself, in a place with top quality mains power. I'm not sure about how would these perform with lower quality power, possibly they would last less, or little (the ones I use last several years here). I don't have experience with UPSs.

This link is for the US but maybe there're or can be similar products in any place. Idk how much effective is this.
What is Dirty electricity, and how to remove it from your home? - Environmental Potentials

I did not see anything suspicious in the event log unfortunately.

I did some testing again this afternoon because the problem would occur even if it's daytime. Tried removing 1 ram stick and opened some apps, chrome tabs, etc. altogether and leave it at idle- tried using it too but the problem disappeared. Moments later I tried putting back the ram I removed and tried to play a game. Nothing happened it was working well.

Then around now, 8pm, the problem occurred again. Tried removing one ram stick but that did nothing. Seems the random restart/shutdown at night problem is still there.

Most of the time it's not even restarting by itself, the screen would just turn black and seconds after the light on the corner of the monitor would go from blue to orange and it would just stay there. When the problem occurs, I usually just restart it by myself clicking the button on the Case and if the display won't bite, I would just force shutdown by holding the button then turn it on again.

Next time video goes off while in Windows, try pressing Ctrl-Shift-Win-B, what in theory restarts the graphics driver, sometimes it works sometimes not. Does the computer seem to be working despite the loss of video? This happened me once while doing a backup in Win7 (where that key combination doesn't work), but the case HD led kept on lit or blinking. When it finished, I forced the shut down, booted up and Macrium logs said the backup and verify were successful, and files sizes and times (backup speed) were normal. It was just sporadic, the system is old and has instability issues from time to time but it works well most of the time.

Do you have a discrete gfx card? What's your PSU? The latter is a main suspect imo. I think you could also try a surge protector if you can get one. The ones I use are power strips with switch and surge protection, where I plug computer, monitor and speakers.

4 120mm? fans might mean 3 intake and one exhaust. 1200 rpm fans might be good for intake, but for exhaust you need something more powerful whatever you use for intake (unfortunately, 1200 rpm fans are a kind of standard for new cases, but they don't suffice!). My least consuming desktop only has a 1600 rpm exhaust (and the CPU and PSU ones, the discrete gfx isn't meant for gaming and doesn't have fan).

I suppose you have 2 equal RAM sticks. You don't seem to have one defective (taking out one then the other would detect it quickly), but taking out one could act as a "stress filter". RAM would be a potential "victim" of inefficient case cooling, if so.