I have noticed this also, but I thought it might be because when it is dark I am out of the room for quite some time and do most of my chores when the lights are on. So I think it could be possible my sense of smell is desensitized and they don't seem to smell as much during the day. When i'm not in the room for a long time and then enter, the plants seem so much more smelly.
It is kind of like the 'fresh country' smell. If you visit a farm all you can smell is cow crap, but the farmer doesn't notice it as his nose is desensitized. A few years back I went to my uncles who lives on a farm and stayed for three nights. The first five or six hours I couldn't stand the 'fresh country' smell, but after that I didn't even notice it anymore.
So really in my case i'm not sure if the plants really smell more at night or it's just desensitization. I could be totally wrong and possibly they do release more smell at night. What do you guys think - could it be a possibility?
TGT