A computer program product comprising instructions which, when run on a processing device of a sleep apparatus, said sleep aid apparatus comprising one or more stimulus generators operable to generate user-perceptible stimuli with one or more sensory modalities, and further comprising a physiological sensor to generate sensor data, cause the processing device to perform a method for controlling the sleep aid apparatus, wherein the method comprises: receiving the sensor data from the physiological sensor; determining a respiration phase of the user based the sensor data; providing a first control signal to the one or more stimulus generators to generate a first user-perceptible stimulus to guide a user in matching a pacing of breathing of the user to a cycle frequency of the first user-perceptible stimulus, wherein the first control signal is provided to generate the first user-perceptible stimulus having the cycle frequency and a cycle phase; determining a synchronization status between the cycle phase of the first user-perceptible stimulus and the respiration phase of the user; providing a second control signal to the one or more stimulus generators to generate a second user-perceptible stimulus based on the synchronization status, characterised in that the second user-perceptible stimulus comprises a vibration stimulus, wherein the second control signal is provided to modulate an amplitude of the vibration stimulus based on the synchronization status. sleep aid apparatus comprising one or more stimulus generators operable to generate user-perceptible stimuli with one or more sensory modalities, and further comprising a physiological sensor to generate sensor data, cause the processing device to perform a method for controlling the sleep aid apparatus, wherein the method comprises: receiving the sensor data from the physiological sensor; determining a respiration phase of the user based the sensor data; providing a first control signal to the one or more stimulus generators to generate a first user-perceptible stimulus to guide a user in matching a pacing of breathing of the user to a cycle frequency of the first user-perceptible stimulus, wherein the first control signal is provided to generate the first user-perceptible stimulus having the cycle frequency and a cycle phase; determining a synchronization status between the cycle phase of the first user-perceptible stimulus and the respiration phase of the user; providing a second control signal to the one or more stimulus generators to generate a second user-perceptible stimulus based on the synchronization status, characterised in that the second user-perceptible stimulus comprises a vibration stimulus, wherein the second control signal is provided to modulate an amplitude of the vibration stimulus based on the synchronization status. receiving the sensor data from the physiological sensor; determining a respiration phase of the user based the sensor data; providing a first control signal to the one or more stimulus generators to generate a first user-perceptible stimulus to guide a user in matching a pacing of breathing of the user to a cycle frequency of the first user-perceptible stimulus, wherein the first control signal is provided to generate the first user-perceptible stimulus having the cycle frequency and a cycle phase; determining a synchronization status between the cycle phase of the first user-perceptible stimulus and the respiration phase of the user; providing a second control signal to the one or more stimulus generators to generate a second user-perceptible stimulus based on the synchronization status, characterised in that the second user-perceptible stimulus comprises a vibration stimulus, wherein the second control signal is provided to modulate an amplitude of the vibration stimulus based on the synchronization status.