Patients with the sleep apnea/hypopnea syndrome suffer from impaired daytime function. This has been attributed to both sleep fragmentation and hypoxemia. To help understand which is casual, we studied the effects of sleep fragmentation alone on daytime function. Sixteen normal subjects were studied on two pairs of two nights. The first night of each pair was for acclimatization, and on the second the subject either slept undisturbed or had sleep fragmented with sound pulses every 2 min. Sound volume and duration was titrated to cause a return to theta or alpha rhythm on the EEG for at least 3 s. Study nights were followed by daytime testing of psychometric function and mood and by a multiple sleep latency test (MSLT) and a maintenance of wakefulness test (MWT). Total sleep time did not differ between study nights (400 +/- 20 SD min undisturbed, 396 +/- 24 min fragmented; p = 0.6). Fragmentation decreased sleep latency on both the MSLT (11 +/- 3, 7 +/- 2 min; p = 0.001) and the MWT (34 +/- 8, 24 +/- 10 min; p<0.001). Energetic arousal (22 +/- 4, 19 +/- 4; p = 0.005) and hedonic tone (29 +/- 4, 27 +/- 4; p = 0.05) decreased after fragmentation. Fragmentation impaired daytime function adjudged by the Trailmaking B (p = 0.05) and PASAT 4-s tests (p<0.03). One night of sleep fragmentation makes normal subjects sleepier during the day, impairs their subjective assessment of mood, and decreases mental flexibility and sustained attention.