Be aware that the brain is a power-hungry/power-intensive organ. Give the brain (and body) enough nutrients they need for functioning. The brain (and the body) enters a power-save mode if not fed properly, which may manifest as reluctance to work (to perform deeper thought) and result in procrastination and worse, in depression. Don't try to replace real nutrients with coffee or other stimulants - these work great on top of a great nutritional base. Be aware that your body has various nutrient buffers, cycles and momentums and it may take months or even years to really run out of some really important nutrients, given your diet and lifestyle, and this will be exacerbated and accelerated by aging. Other nutritional deficiencies can have a fast onset, triggered by a temporary change in diet (something "bad" you ate), or by an illness. Know, that the decline in the efficiency of your digestive tract due to insufficient diet also has its own momentum, so it may be much slower to re-feed the body all the nutrients you need, because the internal organs need to heal first.

Two of my personal "works for me" tips: explore the ketogenic diet and the effect of ketones/ketosis on the brain function, and/or try out the Wim Hof breathing method.