how long is access token valid? 24 hours? or does it expire at midnight?

I know that access token is valid for a day.but does it mean 24 hours from the time the access token is got or does it expire at midnight that day.once,i authenticated and got access token around 2 pm and next day i logged in around 1.30 pm.the program threw error and i had to authenticate again.Please advise.

Thanks and Regards
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