The option for the computer to hibernate is to save battery power and lengthen battery life. However, hibernation requires memory space so that it may retrieve files you were working on before the computer went on hibernation. Hibernation or sleep mode basically means that the computer is asleep.
Hibernation is enabled as per the settings you have set in terms of the duration after which the computer hibernates. You can also enable or disable hibernation completely.
Follow this step by step tutorial to learn How to modify hibernation settings in Windows 7.