Diet, sleep, and exercise are needed in combination. The highest performers across many professions tend to sleep the most. Exercise helps make better use of the foods you are eating and produces the hormones you need to think clearly.
As for supplements to help, examine.com has a bounty of information: https://examine.com/topics/Cognition/
This cannot be stressed enough. I have started to sleep at least 7.5 hours each day. But most of the days it is proving to be difficult and I am able to complete only 6 hours. Still having a routine comfortable sleep has worked wonders for my mood and productivity.
If you have trouble sleeping, try going cold turkey on coffee. It will suck for a couple of days, but you'll feel better.
Once you get used to it, turn it back on sometime temporarily. It's incredibly blinding.