While the effects on sleep and well being of transportation noises are rather well documented, much less is known of effects on sleep caused by low frequency noise (<200 Hz). Low frequency noise is common in domestic environments and originates mainly from ventilation and air-conditioning units, sounds from the laundry room and amplified music. Several case studies and some epidemiological studies indicate that low frequency noise affects our sleep, particularly with reference to the time taken to fall asleep and tiredness in the morning. The auditory system is permanently alert even during sleep, and noise during sleep can lead to a release of different "stress" hormones, without the person waking up. One of our more important hormones related to stress is cortisol. Previous studies have shown that the amount of cortisol in saliva increases after awakening with a peak at about 30 minutes, after which it declines. A previous experimental study showed that the cortisol response after wake up was reduced following low frequency noise exposure during the night. The present study is an extension of the previous study, comprising a larger number of subjects and an extended period of quiet acclimatisation nights. The project was carried out at the Department of Environmental Medicine at Göteborg University, and the completion of the analysis done at the Department of Acoustics. The study was funded by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS).