Does sunlight damage helmet? Lately I am wearing my helmet also on my daily commute to work. 

Of course I don't wear it while sitting at my desk, and it sits on a drawer next to the window, where in the afternoon it is exposed to sunlight with little or no air flow. 

Can this lead to long term damage of the helmet? The helmet shell is white.
I was able to find one study that comes close to addressing this: Age Does Not Affect the Material Properties of Expanded Polystyrene Liners in Field-Used Bicycle Helmets. You'd need to pay to read the whole thing, but the abstract and title are clear enough. It's not clear from the abstract whether the helmets tested had been exposed to UV or not.

According to the Bicycle Helmet Safety Institute, UV exposure is not a big concern because helmets are treated with UV inhibitors, and unless there's visible signs of cracking, talk of replacing helmets on a regular schedule is marketing hype.

The EPS foam makes up the bulk of the helmet, of course, but it's not the only part, and if the shell, pads, or straps are damaged or degraded, that might be a reason to replace it.