There isn't one that I know of... What time of day are you flying at?

Hmmm.... No, no ideas really to be honest!  welcome to the forum BTW... hopefully we can be more helpful on other questions! lol  Have you tried all the different seasons? Does it happen in any other aplications, or just FS? Becoause if it does it could be your GFX drivers.... Have you changed them at all recently?
Dan

Your welcome... and always will be!

What graphics card are you using, Jake?

Because for most graphic card and drivers there is an option to adjust the disply brightness in the video adapter, and not in the monitor.

Assuming you're using XP, for other Windows versions I thiink the procedure is basically the same:
Right Click on Desktop,
Click on properties,
Go to the Settings tab
Click Advanced
Now,  most often there is a tab apart from the five windows ones(General, Adapter, Monitor, Troubleshoot, Color Management) that is specific to the video adapter. 

In my case, this extra tab is called Intel(R) Extreme Graphics, and it has got a small icon on it depicting an LCD monitor.  There is a button called Graphics Properties, and clicking it pops up a new dialog window with a whole bunch of new options, among them the option of controlling gamma, brightness and contrast. But this is only in my case, your display adapter probably is different. But look for any extra tabs in the "Advanced..." dialog window!

Cheers!