1. Conclusion CPAP withdrawal impairs driving simulator performance at a 2h drive sooner than restricting sleep to 5h with CPAP. Overall there were more incidents following sleep restriction but the earlier impairment in the CPAP withdrawal condition suggests a shorter safe driving time. Participants had insight into this increased sleepiness reflected by the higher KSS reported following CPAP withdrawal during the first hour. Results are in contrast to Yang et al. (2006) who report a residual effect of CPAP following on night withdrawal, reaction time tests maybe be to short to accurately identify impairment. In the practical terms of driving any one incident could be fatal. The earlier impairment reported here demonstrates the potential danger of missing just one night’s CPAP treatment and highlights the benefit of CPAP treatment even when sleep time is short. OSA patients should be informed of the potential implications of missing treatment when they start on CPAP. Details of driving impairment following CPAP withdrawal compared with a normal nights sleep with CPAP treatment are published in Filtness et al. (2011) Introduction Continuous positive air pressure (CPAP) is a common treatment for obstructive sleep apnoea (OSA). Untreated OSA sufferers are at greater risk of having driving incidents than healthy people (Ellen et al. 2006). Once treated with CPAP driving performance is no longer impaired and patients are legally permitted to drive. Seep restriction and missing one night's CPAP treatment are possible scenarios faced by OSA patients, who must then assess their own fitness to drive. It is unclear if missing only one night of CPAP will effect driving performance as studies report conflicting results of maintained (Yang et al. 2006) and impaired (Kribbs et al 1993) reaction time performance. This study aims to assess the impact of CPAP withdrawal on the complex task of driving using an advance driving simulator. One night CPAP withdrawal impairs performance at a driving simulator task faster than sleep restriction to 5 hours with treatment in OSA patients A. J. Filtness 1,2*, L.A. Reyner 1, J.A. Horne 1 1.Sleep Research Centre, Loughborough University, Leicestershire, LE11 3TU, UK 2. Accident Research Centre, Injury Research Institute, Monash University, Clayton, 3800, Australia Corresponding author: ashleigh.filtness@monash.edu Summary  Driving performance is impaired earlier in a 2h driving task following CPAP withdrawal compared with sleep restriction.  Subjective sleepiness levels reflect driving impairment suggesting insight into sleepiness.  Short CPAP treated sleep is better than longer untreated sleep in terms of onset of driving incidents. Method Participants: 11 CPAP treated participants (50-75yrs) Protocol: Participants drove an interactive car simulator under monotonous motorway conditions for 2 hours on 2 afternoons, following;(i) normal night's sleep with CPAP (not presented) (ii) sleep restriction (5h), with CPAP (ii)normal length of sleep, without CPAP.. Measures: Sleep length was verified using actigraphy. Driving incidents were noted if the car came out of the designated driving lane. EEG was recorded continually and spectrally analysed for alpha and theta activity. Subjective sleepiness was reported every 200 seconds using the Karalinska Sleepiness Scale (KSS). Analysis: Data for all measures were analysed in 30 minute epochs using a 2 (sleep condition) x 4 (time point) repeated measures analysis of variance (ANOVA). References Ellen, R.L.B., Marshall, S., Palayew, M., Molnar, F., Wilson, K. & Man-Son-Hing, M. 2006, "Systematic review of motor vehicle crash risk in persons with sleep apnea", J Clin Sleep Med, vol. 2, no. 2, 193–200 Filtness, A. J., Reyner, L. A., & Horne, J. A. (2011). One night’s CPAP withdrawal in otherwise compliant OSA patients: marked driving impairment but good awareness of increased sleepiness.. Sleep and Breathing DOI: 10.1007/s11325-011-0588-8 Kribbs, N.B., Pack, A.I., Kline, L.R., Getsy, J.E., Schuett, J.S., Henry, J.N., Maislin, G. & Dinges, D.F. 1993, "Effects of one night without nasal CPAP treatment on sleep and sleepiness in patients with obstructive sleep apnea", The American Review of Respiratory Disease, vol. 147, no. 5, 1162-1168. Yang, Q., Phillips, C.L., Melehan, K.L., Rogers, N.L., Seale, J.P. & Grunstein, R.R. 2006, "Effects of short-term CPAP withdrawal on neurobehavioral performance in patients with obstructive sleep apnea", Sleep, vol. 29, no. 4, 545-552. Results Figure 1: Driving incidents in 30 minute epochs with standard error bars Figure 2: Subjective sleepiness (KSS) over time Figure 3: Standardised EEG power in the alpha and theta range Driving Incidents There was a significant condition by time interaction [F(6,60)=3.40, p=0.006], out of lane incidents being more prevalent following CPAP withdrawal during the first hour of driving compared with sleep restriction. Subjective sleepiness At the start of driving participants felt sleepier following CPAP withdrawal but by the end of the task KSS levels were similar for both conditions, demonstrating a significant condition, time interaction [F(3.94,39.41)=3.39, p=0.018, ε = 0.66] EEG There was a non significant trend for combined alpha and theta activity to be highest throughout the drive following CPAP withdrawal. Increasing combined alpha the theta activity power is reflective of heightened sleepiness.