Each person's brain has a set number of hours of sleep that is required for proper functioning. Don't listen to your parents, co workers or boss telling you that a human only needs 4-6 hours of sleep. Less sleep over long period can lead to poor memory, mental health issues and even Alzheimer's
For example, I require 7 hours of sleep. On days where I sleep less. I'm annoyed, my memory and concentration ability is affected. I feel mentally sick through the day. Once I went I few days like this and then one day I had a good sleep. I realised how important sleep was. Your brain functions so much better. Everything is more clear.
Just pay attention to how you perform on less sleep to understand this.
There are many studies showing association of poor sleep with dementia and Alzheimer's.
There are studies that showing association of poor sleep with high blood pressure and cardiovascular diseases.