Are you staying hydrated?
If you are experiencing difficulty concentrating, headaches and tiredness, you may be dehydrated. Breathing, sweating and using the toilet all result in water loss and so you need to top up regularly. 6-8 glasses/cups daily should be your target though more will be required in hot weather, after strenuous exercise or in air-conditioned environments.
Staying hydrated, especially during the work day can be a struggle for some people so here are some tips to help: