Some light does get in – not much but if you’re particularly sensitive to it, it’s enough to notice
There are other queues they can use too like changes in temperature to let them know it’s night time
They also have internal body clocks that work on a 24 hour cycle that give them active (night) times and inactive (day) times
Often with nocturnal it can also be used to refer to a preference for darkness, not necessarily just night time, so they can adapt and will be more active in whatever time is darkest which will most often be (but might not necessarily be) night. For example: If you have a basement that you use (and turn the lights on) during the night but don’t during the day time you might find that some “nocturnal” spiders will become more active during they daytime darker period.
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