Someone who sleeps during the day
What is an appropriate word for people who can’t sleep at night but sleep peacefully in the daytime?
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A difficulty or inability to sleep at night is known as insomnia, and one who suffers this condition is an insomniac. Sleeping during the day and remaining active at night is known as "nocturnality". Nocturnal only applies to creatures who only sleep during the day as their normal circadian sleep pattern, and therefore doesn't apply to humans (although it is sometimes used colloquially).
I think that what you're looking for is delayed sleep phase disorder.