Your brain gets tired and can lead to less productivity when awake, also sleep relieves stress. Of course, if you got no sleep for an extended period of time your brain would shut down and you would die of exhaustion. Hope this answered your question!
When the body does not rest enough, the hormones respond by increasing the hunger drive. What's worse, the hunger cravings are for the for the quickest most macronutrient dense food available. Sleep is as important as diet!