In order to manage Quality of Service (QoS) and energy efficiency in the optical access network, an online Dynamic Bandwidth Allocation (DBA) algorithm with adaptive sleep cycle is presented. This DBA algorithm has the ability to allocate an additional bandwidth to the end user within a single sleep cycle whose duration changes depending on the current buffers occupancy. The purpose of this DBA algorithm is to tune the duration of the sleep cycle depending on the network load in order to provide service to the end user without violating strict QoS requests in all network operating conditions. In conditions of increased network load, the DBA algorithm initiates a decrease in the duration of the sleep cycle, which results in less energy savings. Under low load conditions, an increase in the duration of the sleep cycle results in significant energy savings, while QoS is still maintained.