You don't get the option in the UI: there's just "Sleep", which on a desktop Mac seems to suspend to RAM only. At least, it seems to on mine, as I found out when I unplugged it.

(Apparently the laptops do a suspend to disk depending on battery remaining, so the OS does support it. It'd be nice to have it available! The disk space required could become annoying if you only bought the 1 TB SSD for your fancy 512 GB Mac though.)