It is useful if you're sick, or if you cannot sleep well at night. It is not recommended for healthy people because it is too easy to fall asleep. The secret to staying awake is to remain absolutely still. Lie on the floor with just a carpet or futon. Before very long you will probably get some painful sensations to focus on and keep you awake. Lie on your right side, and do not move a muscle. Sweep your attention down the body from head to foot, noting the main points of contact. Otherwise, note the rising and falling of the abdomen, or the touch of the breath.
While in Burma, due to the extreme heat, I used lying down meditation as it was a little cooler and less stressful during the heat of the day. With only a straw mat and a hard wooden bed to lie on, it was fairly easy to stay awake for an hour or two, but it was not an easy option.
Be careful that “lying down” meditation doesn't become “lying” meditation.