There are many factors that may contributing to your tiredness, including lack of proper sleep (7-8 hours per night), a sleep disorder (e.g. narcolepsy, insomnia), an underlying illness, or Chronic Fatigue Syndrome (CFS).
Chronic Fatigue Syndrome (CFS) occurs much more often in women than men, and can be found in individuals of all ages. However, diagnosis of CFS is often difficult, since many other illness often cause similar symptoms. Nevertheless, individuals with CFS often experience extreme exhaustion for at least 6 months, which does not go away even after bed rest. Other signs include short-term memory difficulties and flu-like symptoms (e.g. joint pain, feeling unrefreshed after sleep, headaches, sore throats). According to the NIH, approximately 17 million people worldwide (1-4 million in the U.S.) suffer from CFS. For more information on chronic fatigue syndrome, visit http://orwh.od.nih.gov/CSF%202011/myalgic.html.
Narcolepsy, on the other hand, is classified as a sleep disorder. Individuals with narcolepsy experience feelings of extreme sleepiness, often resulting in them falling asleep multiple times throughout the day. For example, a person may find themselves feeling extreme drowsiness every 3-4 hours, causing them to uncontrollably fall into a sleep state. This can even occur while in the middle of a conversation with another person. Unlike in CFS, a person with narcolepsy often wakes up from one of their "sleep attacks" feeling refreshed. Narcolepsy is often confused with insomnia, which is difficulty getting to sleep or staying asleep. For more information on narcolepsy, visit the NIH PubMed Health Article at: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001805/.