To explore the relationship between sleeping disorders(i.e. early awakening disorders)and the P300. Method 22 depression patients with early awakening; 20 non-early awakening and 20 healthy volunteers. Test relevant potential P300 of events and Hamilton depression rating scale. Results Compared with the control group, the group with early awakening has extended P2, N2, and P3 latency and reaction time (p<0.05); conversely, their amplitude were significantly lower. Compared with the control group, obviously, the group with non-early awakening has extended N2 and P3 latency and reaction time (p<0.05); N2 P3 amplitude was significantly lower. Compared with the group with non-early awakening, the group with early awakening obviously has extended P2, N2and P3 latency and reaction time (p<0.05); P2、N2 and P3 amplitude was significantly lower. The score of HAMD of the group with early awakening is higher than the group with non-early awakening (p<0.05). Conclusion Damage to cognitive function exists for both early awakening and non-early awakening group. The extent of damage for the group with early awakening is greater than that for the group with non-early awakening.