It is curious how incomplete at present is the physiologist’s knowledge of both the actual condition of the brain in sleep and of the immediate causes which produce that condition. It is probably true (though it is disputed) that the brain becomes pale during sleep, owing to a contraction of the blood vessels, and that the inactivity of the brain arises from this condition. But it is not obvious what determines the contraction of these vessels at the definitely recurring period of sleep. It is probable that the nervous tissue of the brain is, as are the muscles of the body, poisoned or choked (as it were) by the chemical products of the day’s activity, and so readily cease to be active until the injurious products have had time to be carried away by the blood stream. Muscular substance undoubtedly is affected in this way, and that great muscle the heart, though never resting for a lengthened period, rests after each pulse or contraction, and recovers itself in the brief interval. It is also probable that the exhaustion by the day’s activity of the oxygen stored up in the various tissues of the body produces a condition of quiescence whilst the store is replenished. Stimulation of the nerves through the sense-organs of sight, hearing, and touch will prevent or retard this natural quiescence, and the cessation of that stimulation is favoured first of all by the darkness of night and by the closing of the eyelid, as well as by the removal of clothes which more or less irritate the skin; also by the would-be sleeper taking up a position of perfect rest, and by the exercise of his will, withdrawing his brain as much as possible from all external influences. The would-be sleeper also controls, when possible, that internal stimulation of the brain which we call attention. It is the failure (owing to unhealthy conditions) to control the latter which leads to the most serious kind of sleeplessness, when the brain gets for hours out of restraint and works incessantly like an independent existence. The disturbance of the nervous system set up by irritation of the digestive organs, whether accompanied by pain or not, is an independent cause of sleeplessness which often co-operates with the first, and is (through the mechanism of the nerves) often set going (though it may arise independently) by an unhealthy excess in the excitement of the brain’s activity. There is no panacea for sleeplessness; the only thing to do is to consult a first-rate physician, and strictly follow his advice.