Method of transmission by a watch of an information message relating to an assessment of the sleep quality of a user of said watch, the method including the following steps: - determining the start of the user's sleep period: - from a lighting measurement provided by an environmental sensor of the watch, the start of the sleep period being identified once a low lighting condition has been determined by the processing unit of the watch, and when the lighting measurement transmitted by the sensor is below a reference light intensity threshold during a set time interval; or - from physiological measurement data for the user and/or behavioural measurement data therefor; or - from the use, by the processing unit, of additional information such as the time of day and an estimation of the circadian rhythm of the user; - in response to the determination of the start of the user's sleep period, recording, by the processing unit, data describing at least one event-driven episode of at least one type of environmental event recorded during said sleep period of the user; - identifying at least one type of environmental event that disturbs the user's sleep by processing said descriptive data; - estimating a sleep quality assessment index for the user as a function of a sleep disturbance indicator for each identified type of sleep disturbing environmental event, wherein, during this step, the processing unit determines the assessment index for the quality of this sleep by computing an average of the one or more sleep disturbance indicators obtained in a sub-step of generating a sleep disturbance indicator for each identified type of sleep disturbing environmental event, during which computation coefficients can be applied to certain indicators as a function of the nature of the type of sleep disturbing environmental event to which they relate; and - devising an information message comprising the estimated assessment index for the purpose of diffusion to the user. - determining the start of the user's sleep period: - from a lighting measurement provided by an environmental sensor of the watch, the start of the sleep period being identified once a low lighting condition has been determined by the processing unit of the watch, and when the lighting measurement transmitted by the sensor is below a reference light intensity threshold during a set time interval; or - from physiological measurement data for the user and/or behavioural measurement data therefor; or - from the use, by the processing unit, of additional information such as the time of day and an estimation of the circadian rhythm of the user; - from a lighting measurement provided by an environmental sensor of the watch, the start of the sleep period being identified once a low lighting condition has been determined by the processing unit of the watch, and when the lighting measurement transmitted by the sensor is below a reference light intensity threshold during a set time interval; or - from physiological measurement data for the user and/or behavioural measurement data therefor; or - from the use, by the processing unit, of additional information such as the time of day and an estimation of the circadian rhythm of the user; - in response to the determination of the start of the user's sleep period, recording, by the processing unit, data describing at least one event-driven episode of at least one type of environmental event recorded during said sleep period of the user; - identifying at least one type of environmental event that disturbs the user's sleep by processing said descriptive data; - estimating a sleep quality assessment index for the user as a function of a sleep disturbance indicator for each identified type of sleep disturbing environmental event, wherein, during this step, the processing unit determines the assessment index for the quality of this sleep by computing an average of the one or more sleep disturbance indicators obtained in a sub-step of generating a sleep disturbance indicator for each identified type of sleep disturbing environmental event, during which computation coefficients can be applied to certain indicators as a function of the nature of the type of sleep disturbing environmental event to which they relate; and - devising an information message comprising the estimated assessment index for the purpose of diffusion to the user.