iPhone Usage Bars Gray

I think you are referring to the wrong gray bar. There is a gray portion of the bar over the W for Wednesday and I can click on that but what I am referring to and what I have seen online is no one has provided an true answer for the gray bars that are over the individual hour marks. The phone shows it was used from Midnight though 10am 100 percent of the time as indicated by the full gray bars. I have no idea why the phone was being used constantly. I checked the previous day and there was zero usage between 11pm and midnight on the preceding Tuesday so something happened with the phone exactly at midnight and the phone started showing usage. I want to know what apps were using the phone processor for 10 hours straight.