that may be because you actually oversleep. unless you're very young (<20yo), 7h of sleep is likely all you need, and anything less than 6h or more than 8h will leave you tired and lacking energy.
I could be oversleeping but I cannot really fix that. I don't use drugs to sleep or wake, and I don't wake via an alarm most days (alarm is for 7, I wake up at 6:40~55).
I could set an alarm for 5 am but I predict that I would be even more tired compared to my 10 to 7 schedule.
No source. What he said is just broscience. You will feel tired and groggy if you wake up from an alarm in the middle of a sleep cycle regardless of number of hours slept.
