How to stop sleepy in the afternoon

Generally speaking afternoon fatigue is from what we call Spleen Qi Deficiency in Chinese Medicine terms. The page linked to above contains some commonly used points for that deficiency. Your response with just acupressure will be limited compared to a technique such as moxibustion, or even better full acupuncture treatment from a qualified professional. The most common acupressure points for spleen deficiency would be SP 6 and ST 36.