First, I assume you were in bed/asleep during that 6-8am window?
What are you using for a sleep tracker?
How consistent is the timing?
Generally, this means that Sleep can't determine your sleep state. There could be several reasons for this, and knowing where it's getting its data from might help clarify/identify what's going on.
For example, if you're using a sleep tracking app that uses sound level monitoring, there may be other sounds that are disrupting the monitor - your partner, pet, or even something like heating/air conditioning coming on if the sensor is in the direct airflow.
If you're using a tracker that uses vibration detection, something may be triggering the sensor - maybe your dog jumping on the bed, maybe you or your partner rolling over and it can't distinguish between the different people.
None of the above should apply if you're using an Apple Watch. If that's the case, the most obvious cause would be a loose watch strap, where the sensors on the back of the watch aren't in contact with your skin, but that doesn't explain the timing anomaly.
If you're using multiple sources, it could be they're providing conflicting data and therefore Apple Health isn't able to make a determination as to your sleep state.
Worst case scenario is some health-related issue, which might warrant a talk to your doctor and a sleep study.