A wireless device operates in a first mode in which the device can send data to and receive data from an access point. The device receives control data from the access point comprising first data indicative of a time period and second data indicative of a control parameter. The device operates in a second mode for the time period specified by the first data and on the basis of the control parameter. In the second mode, at least some circuitry of the device used for sending data and at least some circuitry of the device used for receiving data is placed in a low power state, and data to be sent to the access point is stored in a buffer of the device. After expiry of the time period, the device operates in the first mode to send the data stored in the buffer to the access point.