Poor sleep quality and daytime sleepiness have been reported to be associated with poor academic performance , cardiovascular events, and motor vehicle accidents. This study assessed the association between sleep quality and academic performance in Sudanese medical students.
We found a significant difference between excellent and average students regarding overall sleep quality. This result is similar to the study conducted by Seblewengel et al.  in Ethiopia in which significant variation was found between the good sleeper and bad sleepers as regarding all components of the Pittsburg Sleep Quality Index except for sleep latency. The present study reported a significant difference between average and excellent medical student with regards to sleep latency, in contrast to the above study.
Our data showed a highly significant difference in sleep quality (as measured with PSQI) between excellent and average medical students and is in agreement with studies conducted among college students by Aluoja and Tavares [6, 13] who found a significant negative correlation between global sleep quality score and grade point average supporting the hypothesis that bad sleep quality is associated with poor academic performance.
Poor sleep is associated with lack of concentration and inability to function during the day  that affect academic performance. In the current study, sleep duration ranged from 3 to 8 h with a mean of 7.00 ± 1.9 in the excellent group, and 6.30 ± 2.0 in the average student group, and a significant difference was found between the two groups of medical students (P = 0.047). Similar to our result, is the study conducted in Saudi Arabia by Bahammam  who concluded that decreased nocturnal sleep time is negatively associated with academic performance in medical students. Furthermore Bahammam demonstrated that, late bedtime on weekdays and weekends is associated with lower academic performance, and going with the present finding in which, the excellent student weekdays bedtime was earlier than average students, 11.54 pm ± 1.54 h versus 1.12 am ± 1.42 h in average students (P < 0.001), and F = 15.628. The excellent student’s weekend’s bedtime was also earlier than the average students, 12.54 am ± 3.36 h, and 2.48 am ± 2.12 h respectively.
In the current study, students delayed their bedtime by 1.2 h in excellent students, and 1.6 h in average students, similarly a study published by Colette et al.  showed that university students delay their bedtime by 1.5 h.
The outstanding result of the present study is that 61.4 % of the study group had bad sleep quality and is higher than studies conducted in Malaysi  in which 20 % of medical students had bad sleep quality and Estonia which showed that 6 and 1 % of medical students were poor and very poor sleepers respectively .
Due to demands of the academic environment, most medical students are involved in the late- night study and excessive internet use and some also consume stimulants to stay awake at night . The present data showed that; the majority of the average student study group (69.2 %) goes to bed later than midnight during weekdays; similarly a study conducted in Lebanon among university students and concluded that 89.9 % of the responders went to bed after 11 pm on weekdays .
Limitations of the study
The current results are appropriately interpreted with certain caveats. The present study was conducted at only two colleges, so generalizability cannot be ensured; further multi- center larger studies including students with all grades focusing on those with grade D (fail) are needed. Comparison between public and private medical schools with different living environments and class timetable can also be done. The reliance on the self-administered questionnaire to collect data is subjective as students may not have accurately reported their sleep habits. Research suggests that people may overestimate or underestimate self- reported values.
Being a cross-sectional study we cannot determine a causal relationship between sleep quality and academic performance. In the present study we excluded those with grade D (failing) which is the most important group to focus on; also we were not able to control for potential confounders like stress and class attendance. Experiments conducted on humans showed that sleep deprivation leads to impairments of memory, concentration and performance .
Due to the growing concerns about the association of poor sleep quality with many physical and mental diseases, it is imperative to conduct additional research to examine potential causes, and implement appropriate preventive measures and treatment when needed.