There are many reasons why you may feel sleepy and tired after working out, but here are some suggestions that may help you.
1.Sleep at least 8 hours a day. If you happen to have an iphone, there is an app called "sleep cycle" which will help you being more lucid and energetic throughout the day.
2.If you can, get a trainer to help you make an exercise plan which is adapted to your current performance qualities. This will help you keep going to the gym.
3.Keep plenty of water with you after exercising to avoid exhaustion.
4.Eat a consistent breakfast every day. 2 pieces of fruit, cereals, low fat.
It's also worth mentioning that if you have not exercised for a while or haven't had a very active lifestyle, your body needs to get used to your new energetic expenses. In other words, once you burn a certain amount of calories regularly, you won't feel depleted anymore.