When you boot it back up again, it has to go through the typical boot-up process, initializing hardware drivers, loading startup programs, and so on. When you shut down your computer, it discards the current system state and powers off.

Because the computer doesn’t have to provide power to its RAM, Hibernate mode uses less power. This may or may not be faster than a normal boot-up process, but it allows you to maintain your system state including all your open programs and documents. When you use Hibernate mode, your computer saves the contents of its RAM to its hard drive and “powers off.” When you boot it back up, it will read the previous contents of the RAM from the hard drive and restore them to RAM. While in sleep mode, desktop computers and some laptops have an LED that stays on, so that’s another source of power consumption. Sleep mode continues providing this power, allowing a sleeping PC to wake up almost instantly. The RAM contains your computer’s working memory and requires constant power or it will forget its contents. When you use Sleep mode, your computer continues providing power to its RAM. RELATED: What's the Difference Between Sleep and Hibernate in Windows?

Computer not staying in sleep mode