Based on my experience:

Apart from number of hours, it is also important that you sleep sound. 7 hrs of sleep for a person with no sleep disorder and person with sleep apnea has different impact on productivity.

For me this is what I have found age < 20: Sleep hours never impacted my productivity, even if awake all night. 20-30 Years of age: Min 5 Hrs of sleep and I was 100% productive. 30-35+: Productivity got impacted if I slept less than 6. At present(35-40): I am 100% ONLY if I sleep more than 7.5 Hrs.

Every now and then we see studies come out and researchers tell us the importance of sleep and its impact on our brains. I found it to be true in my case.