The 2D SST has a high temporal resolution and analyzes behavior as a continuum, rather than discrete states, and so facilitates higher dimensional examination of state transitions. Traditional spectral analysis often excluded or diluted events through averaging. In addition, this approach employs the ratio of two frequency bands rather than a single frequency band. We determined whether UAO animals have state instability reflecting abnormal sleep/wake states, faster movements between states, abnormal transition processes, and fragmented sleep. The general location of state space clusters SWS and PS are conserved in UAO rats. However, we identified several differences between groups. The density graph analysis indicates that the SWS cluster did not change between control and UAO, while the wake cluster shifted to lower ratio 1 in the UAO group. The UAO group has a higher velocity at all regions of the 2D state space plot, suggesting less stable vigilancestates. UAO leads to more trajectories between wake and LSWS and vice versa and higher microarousal index, indicating that obstructed animals have fragmented sleep. In children it was reported that early adenotonsillectomy leads to significantly larger RU 24969 hemisuccinate decrease in the arousal index and in the percentage of sleep time in stage N1, consistent with improved sleep continuity. Transitions between wake and SWS in UAO rats do not originate in extreme regions of the deep SWS and PS. UAO animals spent less time in typically ����stable���� areas of wake and SWS. This reduction in delta-rich SWS in UAO is consistent with the reduction in slow-wave activity in rats and in humans with sleep-disordered breathing. Administration of ritanserin has a strong sleep consolidation effect in both groups, similar to earlier reports in rats and in humans with preexisting sleep fragmentation. Similar to other reports, the effects of ritanserin on sleep/wake pattern are limited to the first hours of light onset following drug administration due to its known pharmacokinetics. The improvement of sleep-wake activity in our study following ritanserin was due to increased time spent in stable regions of the DSWS cluster, less fragmented sleep, and TC-S 7004 decreased number of microarousals from LSWS. Ritanserin, which has a role in regulating SWS depth, stimulates hypothalamic growth-hormone-releasing hormone secretion in UAO. It is possible that up regulation of hypothalamic orexin in UAO rats has an important role in this sleep fragmentation. SST of the EEG is especially useful for studying the dynamics of sleep/wake instability, but it has some limitations.