The priority at this late stage is that you enter the examination hall well-rested, well-fed and with an appropriate level of stress.
1. Sleep early every night
Go to bed before 10pm (or 9pm with an exam the next day)
Wake up naturally. If you’re waking up too late, go to sleep at 7pm.
Avoid backlit screens for one hour prior to sleeping. Backlit screens emit light in the 484-nanometre range, which excites melanopsin in the retinal ganglion cell photoreceptor. This disrupts your circadian rhythm and keeps you awake!
2. Eat healthily
Eat regular meals at regular times.
Eat plenty of fruit. (Five per day.)
Drink plenty of water.
3. Get some lighter exercise
Avoid exhausting sports around exam time (e.g. rugby).
Do more walking, jogging, and lighter sports at exam time (e.g. badminton).
Drink plenty of water(!) Aim to drink 3 litres per day.
Research has shown that you perform difficult tasks (such as a Chemistry exam) much better under moderately relaxed conditions. The famous Yerkes-Dodson curve illustrates this beautifully.