OS X Yosemite: About FileVault encryption
To ensure security when you turn on FileVault, other security features are also turned on. For example, when you turn on FileVault, you need a password to log in when your Mac is in sleep, or after leaving the screen saver. After the initial startup, only users enabled in FileVault can log in; other users need an administrator to log in first. For more information, see Encrypt the contents of your Mac with FileVault.