In fact, it's not actually necessary to reboot in order to reach single-user mode. The init program manages the current mode, or "runlevel", for the system. The standard runlevels for a Linux system are labeled and defined as follows:

• 0: Halt the computer
• 1 or s: Single-user mode
• 2: Multi-user, no network
• 3: Multi-user, text console
• 4: Multi-user, graphical console
• 5: Same as 4
• 6: Reboot the computer

These runlevels vary between distributions, so be sure to consult your distro's documentation.