struggling to maintain concentration after 3+ hours into a game.
This is not something that you can fix with chess exercises. This is a physical problem.
I had something similar a few years ago although I'm embarrassed to admit that it kicked in after about 30 minutes! During our winter club competitions you typically get between 40 and 50 players crammed elbow to elbow in one large room with the windows shut. I was unfit at the time and if my opponent inconsiderately had a long think 10 or 12 moves into the game I found myself nodding off from the warmth and lack of oxygen in the room.
Round about the same time I read a (junk science) newspaper article saying that dieing to go for a pee improved concentration. I came up with the combination solution of starting the game with a paper cup of cold water, sipping regularly and going for a refill when it was empty. This meant that every 10 or 15 moves I was getting up to go to the kitchen where I could open the window and take a few deep breaths of cold air as I refilled my cup. Later in the game I was making regular trips to the toilet ;-) where again there was an open window and fresh air.
Now I'm fit again and don't have the same problem.
Another approach is to look at cold remedies. The one I take when I have a heavy cold or touch of flue has the following active ingredients: paracetamol 300mg, caffeine 25mn, phenylephrine hydrochloride 5mg. Make of that what you will.
I would, however, point you towards the FIDE anti-doping policy . In particular if you are close to the Australian women's team note this section:
The most relevant banned substances for chess are:
• Amphetamines – e.g. Adderall, Ritalin
• Ephedrine and Methylephedrine – Prohibited by WADA when its concentration
in urine is greater than 10 micrograms per milliliter
• Pseudoephedrine is prohibited when its concentration
in urine is greater than 150 micrograms per milliliter
Substances not present on the Prohibited List but represented in the
• Caffeine – Included in WADA 2013 Monitoring
Program and relevant for in-competition testing only. Any test reading
of less than 400 milligrams poses no problem.
• Codeine – A common ingredient in, for example, preparations used
to treat coughs and
stomach upsets. Any dosage is highly unlikely to be significant when
taken in normal therapeutic quantities.
Just how this relates to common cold medication I don't know although a British skier had his Olympic bronze medal taken away from him after taking such a cold remedy.