The effects of strong daytime noise stress on subsequent undisturbed night sleep were studied in six male volunteers. They slept for seven consecutive nights in the laboratory, two nights being preceded by an 8 h exposure to 83 dB (A) pink noise. Continuously during all nights EEG, EOG, EMG, ECG and respiration were recorded. Additionally, during five nights, blood samples were taken every 30 min by an indwelling venous catheter for the determination of ACTH, hGH, PRL, TRP, 5-HT and 5-HIAA. After daytime noise load, increased sleep stage 4 stability, partly elevated hGH and PRL levels and decreased levels of the metabolites of the serotonergic system were found. This result may be explained by the assumption that high daytime noise stress is an additional load for the CNS which demands an intensification of recovery processes during the sleep of the subsequent night.