Sleep score from last night disappeared after nap

I had a sleep score of 76 from last night and took a nap this afternoon for just over an hour. It registered as a duration of 2:08 and gave me a sleep score of 0, wiping out the score from last night. Why did this happen when my sleep settings are set to 10:30pm - 6:30am? I manually edited the sleep times to what I had last night, and now it gives me a score of 48 instead of 76. Any way to prevent this in the future?