Should I sleep whenever I feel tired?

No. You could develop poor sleep habits with possible chronic insomnia at night. If you need to take a nap, limit it to 20 min and look into the causes of your sleepiness.
If you can. Your body knows when it needs rest. Responding to its' messages can be beneficial. Google uberman polyphasic sleep for more information. It consists of many short naps throughout the day and night rather than and eight hour sleep.