I had read some research paper showing the optimal number of sleep hour is between 6h 30 and 7h 30.

One thing to note is sleep deprivation is really problematic. Don't do this.

But you could sleep far less if you sleep in 3 our 4 times instead of only one time. But one thing to experience is the nap.

Try sleep only 4 to 5 hours. Then take nap at 12:00 for about 30min to 1h. And for me it worked well for some months.