Have you noticed that by default Debian actually does not reboot machine when you try to reboot? Instead it uses kexec to “restart” the kernel.
This is not very useful sometimes, because in some cases if you have multiple operating system installed, and you need to boot one of these, you’d prefer actual reboot really to happen so you can select target OS from grub menu. In order to make this, you need to uninstall kexec-tools package:
sudo apt-get remove kexec-tools
After uninstallation of kexec-tools your reboots will actually become reboots.