A word for one who has the ability to sleep almost instantly

It is common, though possibly insensitive to actual sufferers, to use the term narcoleptic in an exaggerative sense to refer to people who can fall asleep nearly instantly. It is actually more common (in my experience) to see the word used to describe someone like your wife, than to describe someone who is actually afflicted with narcolepsy, as the latter is far more rare. The tone is always playful in this form of usage and does not imply that there is anything actually wrong with the subject.

Alternatively, you could use a term of your own devising that is easily understood such as "sleep-on-demand".