Around 1590, hammocks were adopted for use in sailing ships; the Royal Navy formally adopted the canvas sling hammock in 1597. Aboard ship, hammocks were regularly employed for sailors sleeping on the gun decks of warships, where limited space prevented the installation of permanent bunks.
Since a slung hammock moves in concert with the motion of the vessel, the occupant is not at a risk of being thrown onto the deck (which may be 5 or 6 feet below) during swells or rough seas.
Likewise, a hammock provides more comfortable sleep than a bunk or a berth while at sea since the sleeper always stays well balanced, irrespective of the motion of the vessel.
Prior to the adoption of naval hammocks, sailors would often be injured or even killed as they fell off their berths or rolled on the decks on heavy seas. The sides of traditional canvas naval hammocks wrap around the sleeper like a cocoon, making an inadvertent fall virtually impossible.[original research?] If suitably packed, they could also be used as emergency flotation devices.
Many sailors in the Royal Navy, during the 1950s at least, used a spreader – a length of wood with a V cut in each end to engage the second hammock string on each side. The first string was set up more tightly than the others so that it raised a protective lip along each side to keep out drafts and prevent the sleeper being thrown out.
A narrow mattress was also issued which protected the user from cold from below. In addition naval hammocks could be rolled tightly and stowed in an out of the way place or in nets along the gunwale as additional protection during battle (as was the case during the age of sail).
Many sailors became so accustomed to this way of sleeping that they brought their hammocks ashore with them on leave. The naval use of hammocks continued into the 20th century.
During World War II, troopships sometimes employed hammocks for both naval ratings and soldiers in order to increase available space and troop carrying capacity. Many leisure sailors even today prefer hammocks over bunks because of better comfort in sleep while on the high seas.
Hammocks have also been employed on spacecraft in order to utilize available space when not sleeping or resting. During the Apollo program, the Lunar Module was equipped with hammocks for the commander and lunar module pilot to sleep in between moonwalks.