It is very important to know that it takes a long time to clean your body and mind, especially if you have abused your body with processed junk, meats, alcohol, dairy, etc. No one can tell you how long it will take you to feel better, it is a process.
It takes bad food to get us sick and lose brain cells but fasting with water will actually grow new brain cells, a lot of research has been done on that. Read this...
Also there are many such articles on the internet and videos on YouTube re: this subject. I can attest that my memory is getting better as I age, I fast regularly, been doing it since I was seven but started Linder fasts 14 years ago and have not stopped it. I function much better using my brain during a fast than when eating regular meals.
Sleeping is very good during a fast. Make sure you drink warm water, filtered or distilled, stay warm and rest. No toothpaste, no chewing gum, no supplements or salt should be used while fasting.
Good wishes and great job in getting to five days.