Studies have found that the effects of a cup of coffee or a glass of cola are noticeable after just 10 minutes, but the peak caffeine concentration in the blood occurs after 45 minutes. If you take the same caffeine dose in tablet form, the peak caffeine level will be the same but it takes longer – between 60 and 75 minutes – to reach that peak.
For most people, the caffeine level in your body halves roughly every six hours. So 50 per cent of the caffeine from your 4pm cuppa is still circulating in your system at bedtime.
