HELP - Hot water running out

Hoping to get some help on why the hot water in my master bath doesn't stay hot at night. My house is single story, 1500 sqft, with 2 baths.
I have a 40 gallon water heater, installed in June 2011.
There are 4 people in the house.

I tried running several scenarios to try and figure out what is going on.
Hoping I can get some suggestions here.

The master bath stays hot in the morning, but at night, the hot water will stay hot for a couple minutes, then turn luke warm...almost a little cold.
Thinking my hot water heater was just running out of hot water, I tried taking a shower in the 2nd shower (it's a little closer to the water heater) at night time as well, and it was hot the whole time.

I rarely run the dishwasher at night, and I'm not running a clothes washer.
I give the kids 1 bath around 7pm. My wife showers around 8pm. Sometimes it's already luke warm by this time. So again, i don't think the hot water heater is running out of capacity.

I also removed the anit-scalding valve in my master shower, but no difference.

Is it perhaps the hot water pressure not strong enough?
My pressure was too high, and i had a plumber install a regulator to get it back to normal about 5 months ago.

Any suggestions are greatly appreciated!

I'm not a plumber but I know from personal experience a little bit. I'm having trouble right now with my main shower not getting hot enough, but I know what it is. The cartridge wears over time especially if you have hard water. Sometimes it will allow it to get hotter and sometimes i'll be taking a cool shower when I know there is hot water. Haven't gotten around to replacing it yet but if you're having temp problems that vary from time to time it might be the cartridge behind your shower handle that fits inside the valve. I don't think it's a pressure problem especially if it works in the morning. I'm not sure what would cause it to only happen at night though. But with videos online and schematics of your model shower fixtures you should easily be able to replace the cartridge without a plumber. I'd start there first before you call someone out. A buildup on the cartridge will affect temp over time I know that because I have the problem now.

is it just your shower I. The master bathe or the faucet also??????

Not a plumber either, but I'd go to the source. I'd make sure the water heater is working correctly, then go from there. I'm having a somewhat similar problem, so I'll be interested in reading what the experts have to say.

At his is for both posters if it only occurs in the showers
I don't think it's your hwh clean the stem or cartridge in viagar
some stems are pressure balance some therostatic
some work with hwh some only work with tankless systems