Perhaps the "bloom" around the engines may be more visible at night, and perhaps lights may illuminate reflective particles otherwise invisible during the day........actually I don't know......does anyone?
If you're flying in South and West UK tomorrow night....let me know. I'll be on 134.075/133.6/129.375/135.255/132.95/126.075/128.815......or if any one of those isn't working...127.7 as well.