A wireless device (202) is assisted by an access point (200) to perform contention based access on a radio channel for uplink transmission to the access point (200). The access point (200) obtains (2:1) a traffic load L on the radio channel and determines (2:2) a sleep window based on the traffic load L so that the sleep window increases when the traffic load L increases. The access point (200) then signals (2:3) the sleep window to the wireless device to instruct the wireless device to apply (2:4b) sleep mode according to the sleep window when the wireless device senses (2:4a) that the radio channel is busy. Thereby, the risk for congestion on the radio channel can be reduced during high traffic load and the energy consumption can also be reduced in the wireless device.