An irresistible impulse to jump into the sea, often occurring from the delirium caused from a heat stroke or fever suffered in the tropics. The delirium is usually described as when a sailor pictures the sea as grassy meadows and wishes to dive overboard into them. This may account for many seafarers reported to be "missing at sea". The situation most commonly described is that of the sailor being alone, leaning over the outer rail of the ship, feeling physically tired and perhaps reflecting upon his life ashore. The forward motion of the ship appears to enhance calenture, and is rarely experienced while the ship is stationary. It is said to be most common in hot, clear and calm conditions.
French doctor JP Falret wrote in 1839 that sailors in the 18th and 19th centuries often had to be physically restrained to guard against the furious intensity of this delirious state. A vessel en route to Rio de Janeiro in 1829 was reported to have lost 100 of its crew of 600 by calenture.
Despite, little reference to the phenomenon in the 20th century, a study reported in the British Journal of Medical Psychology in 1983 claimed that the threat was as great as ever. 50% of one ship's crew of 70 had experienced an impulse to jump off the ship. Sailors experienced a desire to jump, to soar or fly from the deck. They spoke about being "lured" and "hypnotically attracted" to the sea. 4% of sea fatalities are in the category "missing at sea".