Studies have shown that it is safe to have caffeine in moderation. For the average adult this is ≤400mg/day (6mg/kg body weight) or 750mL of coffee/1L of black tea. During pregnancy or lactation this is ≤200mg/day or 375mL coffee/500mL tea. For children and adolescents this is ≤2.5mg/kg body weight. Too much caffeine beyond these levels can result in restlessness, anxiety, irritability, muscle tremor, sleep disturbance, headache, increased urine production, abnormal heart rate, sensory disturbances, nausea, vomiting, diarrhoea, seizure, hallucinations, stroke and even death. If you are consuming more than the recommended amount, gradually reducing your caffeine intake can prevent withdrawal symptoms (headaches, fatigue, drowsiness, impaired concentration, mood, irritability).
It is best to have caffeine outside the times of peak cortisol levels (hormone for alertness controlled by our circadian rhythm) to avoid reducing the effect of caffeine. Our peak cortisol levels are generally between 8-9 am, 12-1 pm and 5:30-6:30 pm so it would be more useful to have caffeine at 9:30-11:30, 1:30 or 5:00. Do not have caffeine within 6 hours before you sleep or it could increase the time it takes to fall asleep and decrease your sleep time. Also there is emerging evidence on the negative impact of caffeine near mealtimes on glucose tolerance and insulin sensitivity which is related to diabetes, so have caffeine between meals.
i.e. For the average person have ≤400mg of caffeine per day, have caffeine at 9:30-11:30, 1:30 or 5:00, not within 6 hours before sleep and in between meals!
Source: