Feeling sleepy after sleeping in is a relatively common observation, although sleeping late can have both good and bad effects. For some people who are deprived of sleep during the working week, sleeping late at weekends allows them to catch up on sleep and can be beneficial. It is not uncommon for individuals to complain of being excessively sleepy or tired following this prolonged sleep time. This is called sleep drunkenness, whereby, people have difficulty functioning following waking. The medical reason for this has not been determined, although, disruption of the body clock (circadian rhythm) may be a factor. Sleeping in late may disrupt the body clock and therefore, you may feel sleepy at a time when you are usually quite awake. Another factor may be waking from REM sleep or deep sleep, rather than lighter stages of sleep.
In order to mitigate this feeling, a shower or caffeine may stimulate individuals. However, often exposure to bright light outdoors for 15 minutes or exercise (preferably outdoors in light) will help with this, by resetting your body clock. In general, going to bed and getting up at similar times each day will result in better quality sleep and reduced daytime sleepiness.
For those individuals, who persistently sleep for long periods of time at night and constantly complain of feeling sleepy, review by a family physician may be necessary to make sure that other underlying medical conditions such as sleep apnea are not present.