anapanasati: nimitta or ambient light?

I have started facing a weird problem which I wouldn't earlier. While meditating during daytime, I sometimes doubt whether I am actually experiencing *nimitta* or my *vittaka* has waned and, instead of making way for *ekaggata*, my closed eyes are in fact picking up ambient light. This inevitably keeps me from attaining stable *anapana-nimitta* in that session and the progression plateaus, irrespective of how effortless maintaining concentration has become. That leads to restlessness and I either don't feel like keeping my *aditthana*, or if I can nullify that I emerge from the session somewhat disappointed that I didn't utilize the session well. I'd like to clarify that I not seeking any *jhanic* factor or for *anapana-nimitta* to emerge, and I can attain *anapana-nimitta* and subsequently absorption for desired time period before progressing to the higher *jhanas* during night-time because I know I can't be confounded by ambient light... So, other than only sitting at nighttime, which isn't really a solution, what do I do? Any pointers?