99% of human beings need between 7.5 and 9.5 hours of sleep to be healthy. Fewer than 1% need a little more or less. However, the fraction of that 1% that needs less sleep still needs at least 6 hours of sleep per night. You need the amount of sleep that you need, and there is no changing it without negative repercussions.
You can’t repay sleep debt at all, especially not over the long-term. People who are regularly sleep deprived (that means less than your body needs) don’t function as well cognitively (it makes you dumb), body-wide inflammation, can cause obesity, causes a much higher risk of neurodegenerative diseases, cognitive decline, heart attacks, strokes, emotional and psychological problems, car accidents, cancer, diabetes, wrinkles, and many more negative effects. It may also lead to DNA transcription errors. People who sleep less than 6 hours a day are at least 4 times more likely to die of any cause than those who sleep more than 6. 
When you don’t sleep enough (a minimum of 6 hours), your brain cannot clean itself out. The glymphatic system cleans the toxic chemicals out of your brain as you sleep (and it does its work best when you sleep on your left side). It changes the shape of the cells in your brain, allowing the unwanted waste to leave the brain. 
Besides waste elimination, the glymphatic system also facilitates brain-wide distribution of several compounds, including glucose, lipids, amino acids, growth factors, and neuromodulators. Intriguingly, the glymphatic system functions mainly during sleep and is largely disengaged during wakefulness. The biological need for sleep across all species may therefore reflect that the brain must enter a state of activity that enables elimination of potentially neurotoxic waste products, including β-amyloid (build-up of β-amyloid is linked to Alzheimer’s). 
And there are also things like muscle repair, memory consolidation, and release of hormones regulating growth and appetite that only happen during sleep.
Sleep deprivation can cause your mental state to deteriorate, and if you go for years without sleeping well, you increase your risk for dementia and other neurological disorders. 
It has also been shown that people who are deprived of just one hour of sleep a night for a week are worse at basic tasks, including simple games and driving, than people who have a blood alcohol level of .08. 
New studies show that sleeping less than your body needs suppresses the immune system, which means you are far more likely to get sick. 
Many studies have been done on trying to change the amount of sleep you need. All the studies that were done using sound methodology and practices found that you cannot change how much sleep you need. You may be able to break it up into a couple of shorter periods (biphasic sleep), but you still need the same amount.
For more info on why, see my article here: What Happens When We Don’t Get the Sleep We Need?
 Uncovering Residual Effects of Chronic Sleep Loss on Human Performance
 Sleep Habits: More Important Than You Think
 Sleep Deprivation
 Brain may flush out toxins during sleep
 The Glymphatic System and How Metabolic Waste is Removed from the Brain
 The Glymphatic System: A Beginner’s Guide.
 Human Sleep and Cognition
 Human Sleep and Cognition
 Sleep Deprivation Causes Suppression of Immune System, New Studies Report