People are divided into different chronotypes (categoriation by time of day when they are most active). They can be divided into 'morning people' or 'night owls'. According to a study published in the Sleep Journal a recent study at the Psychiatry and Behavioral Sciences department at Stanford university showed that consumption of caffeine has no effect on 'night owls' but can disrupt sleep for the 'morning people'. The study has only been conducted on students right now and may be too biased since most students tend to be 'night owls' these days. Alsocaffeine elimination from the body differs from person to person.