Environmental factors and bedtime habits
Due to excessive light exposure, mind stimulation, stress, and sometimes some other factors, the mechanisms responsible for your healthy sleep likely got disturbed, which drastically decreased the sleep’s cycles quality.
It’s very possible that if you were to keep your phone and laptop in another room, decrease your stress levels prior to sleep (by reading a book or meditating), yet sleep only six hours, those six hours would regenerate you far better than the seven and a half mentioned above. Long sleep does not mean quality sleep.
Consider number of cycles in a week
What’s therefore important to understand is that the amount of hours we sleep matters only as long as it represents a total number of quality sleep cycles we finish each night.
We don’t need to feel bad after one night of poor sleep. Our body is adaptive and there is some flexibility in the matter - Nick Littlehales argues in the book Sleep, that ‘rather than take a nightly approach it’s important to be looking at our sleep on a weekly basis. Littlehales suggests that the average person needs around 35 cycles of sleep a week - which works out as five lots of 90 minutes a night.