Sensor means for monitoring the awareness status of an individual within a monitored environment (50) comprising means (55) for determining the attainment of at least one predetermined condition identifying the requirement for the awareness of an individual to be monitored, and operative to trigger an alarm (70), means (15) operable by the monitored individual for inhibiting the operation of the alarm, and delay means for delaying the triggering of the alarm (70) for a predetermined period after the attainment of the said predetermined condition.