It could be a lot of things! Low iron/anemia, SAD (winter depression), low thyroid, a virus or infection that your body is using energy to fight, mild dehydration, depression, or some other illness. You need to visit your doctor to get a checkup and blood tests. The blood tests are important to rule out a lot of things. You didn't mention how long you've felt this way, or if it comes and goes.
In the meantime, here are a few things to try:
1) Start taking a Multivitamin and Iron supplement every day. Extra B vitamins, and Omega 3 are very helpful too.
2) As soon as you wake up, get out of bed and drink a whole glass of water (or some juice or milk) before your coffee. Drink more water during the day, and see if it makes a difference after 3 days.
3) Make sure your breakfast has a lot of protein and/or fiber. Sugar or plain carbs can make you sleepy just an hour later. Eggs, high fiber cereal, yogurt, cheese, etc.
4) In case it's winter depression (which is very common where I live), get some sunlight or bright light every day, preferably from 10am-2pm. Find a sunny window, go outside, take a drive - even if you don't feel like it. If you get this way every winter, try a bright light made for Seasonal depression, try a low carb diet (carbs make it worse), and get more exercise.
5) Get more physical activity. I know you don't feel like (believe me, I've been getting bouts of this for 30 years), but in most cases it helps a lot. It might help to pick out 2-3 CDs or songs that never fail to make you tap your feet or move your body. And put them on early in the day. Exercise helps prevent most major health problems, and is as effective in fighting depression as anti-depressants.
6) Don't forget to call your doctor and get some blood work.
You don't have to continue to live like this.
