The invention relates to a system for characterising the sleep of an individual, the characterisation system comprising: a measuring device for measuring a brain activity signal representing the brain waves; a central electronic unit for identifying, in a range of frequencies of between 9 and 16 Hz, a smaller range of sleep spindles of the individual, the smaller range of sleep spindles comprising the brain wave frequencies of the sleeping individual, having an amplitude higher than 15 μV and a duration of between 0.5 seconds and 2 seconds, and comparing, with a threshold, at least one parameter of the brain activity signal of the awake individual in the range of frequencies corresponding to the smaller range of sleep spindles; and a communication interface connected to the central unit and used to emit a warning signal perceptible by the individual when the parameter of the brain activity signal in the range of frequencies corresponding to the smaller range of sleep spindles exceeds the threshold.