Although greater psychiatric symptoms have been reported previously in those with DSPD or evening-type preference, this study is the first to systematically examine differences in the occurrence of psychiatric diagnoses in those with DSPD compared to individuals with just an evening-type preference. We hypothesized that there would be a greater likelihood of an Axis-I disorder in those with DSPD compared to evening-types. However, in this study there were no significant differences in the occurrence or types of disorders between the groups. This would suggest that the occurrence of psychopathology in DSPD is primarily influenced by having an evening chronotype, rather than a sleep complaint (i.e. DSPD) per se, and that the underlying mechanism (s) is an involvement of the circadian timing system.
Although it should be pointed out that sleep is likely to play a role in the prevalence of psychiatric disorders, since both groups had poor sleep quality, but in different ways. While those in the evening-type group did not report insomnia, daytime sleepiness, or problems with daytime functioning due to their sleep, they did have a significantly shorter objective sleep duration, which was mostly likely due to the earlier wake time, and lower sleep efficiency compared to the DSPD group. This was an unexpected finding considering that those with DSPD are diagnosed with a sleep disorder and the evening-types are not. While the evening-types seem to have poorer objective sleep quality, the DSPD group had worse subjective sleep quality (PSQI score) and sleep related quality of life (FOSQ) and a lower Horne-Ostberg score (indicating greater eveningness),than the evening-types. Controlling for sleep duration, subjective sleep quality (PSQI) and sleep-related quality of life (FOSQ) did not change the associations between group and psychiatric diagnoses.
The only significant difference between the groups, after controlling for demographic and sleep variables, was for the evening-type group to have an increased likelihood for having met criteria for >1 disorder in the past. Since none of the sleep or demographic factors by themselves were associated with having more than one past diagnosis. Why then would the evening-types be more likely to have more than one past diagnosis? Perhaps for some evening-types this preference and the associated alterations in sleep manifests not in a sleep complaint as reported by those with DSPD but in psychiatric disorders.
The number of current axis-I diagnosis reported in this sample is considerably higher (~50% current) than that reported in the general population (26.2%, 12 months) (39
). The most common psychiatric diagnoses in this study were mood, anxiety and substance use disorders, similar to those reported previously for DSPD and evening-types. These comparisons should be made with caution however, because our sample was not randomly selected and therefore not a true estimate of prevalence.
A past diagnosis of major depressive disorder (MDD) was the most common mood disorder reported (~40%) in both groups. Of those participants with MDD approximately a third had recurrent episodes. Although the presence of seasonal affective disorder was not specifically examined in this study, review of the time of the year of the reported depressive episodes did not indicate seasonality. This is an important distinction as seasonal affective disorder is thought in some cases to be associated with a circadian phase delay (40
) and an evening-type preference (42
). In the general population the prevalence for lifetime major depressive disorder is about 16.2% (43
), considerably lower than the percentage reported in this sample of DSPD/evening-type individuals. Anxiety disorders were also common among the participants, with specific phobia being the most prevalent, a finding that has not been reported previously in those with DSPD or evening-type circadian preference.
Another interesting finding was the high occurrence of substance use disorder among both groups (current ≤15%). This is in contrast to data from the general population in the USA which reports substance use disorder in the past 12 months at 3.8% (39
). The high occurrence in this sample was largely contributed by the high level of alcohol use disorder. For those who met the criteria for alcohol abuse, approximately 50% of episodes occurred when they were young adults or while in college, a time when alcohol abuse (44
) and binge drinking are common (45
). An alternate explanation is that those with an evening-type diurnal preference, particularly at the age of reported abuse, may be more likely to have poor health related behaviors. For example, previous studies have reported that evening-types may be more likely to smoke (46
) or to use alcohol (48
), sleep medications and caffeine (49
). While there is no direct evidence of this from the current study self medication of insomnia symptoms with alcohol and drugs has been reported in DSPD (29
) and may, in part, explain the number of individuals with a substance use diagnosis.
The underlying mechanisms that link psychiatric disorders with the evening circadian chronotype are unknown. We hypothesize a common link between circadian function and vulnerability for psychiatric disorders. However, alterations in homeostatic regulation of sleep or its interaction with circadian processes, sleep-wake behaviors and sleep disruption (such as the short sleep duration and poor sleep efficiency reported here in the evening-type group) may also play a role in the development of psychiatric disorders. Sleep-wake is believed to be controlled by two opposing processes; the circadian alerting signal and the homeostatic drive for sleep (6
). The homeostatic drive for sleep increases with the time awake, and an alteration in the build up or dissipation of this homeostatic drive could influence sleep-wake timing and behavior. In the current study we did not directly assess the relative contribution of homeostatic versus circadian timing on the relationship between circadian chronotype and psychopathology. In the current study we only measured the timing of the rest/activity cycles. Therefore, a limitation of this study was the lack of direct markers of circadian phase (i.e. melatonin profiles) or homeostatic decay. Recent data indicate that “morningness” and “eveningness” may also be determined by homeostatic mechanisms. For example, there is evidence that evening-types with intermediate circadian phases have lower levels of slow wave activity and slower homeostatic decay of sleep pressure than morning-types with an intermediate circadian phase, interestingly this relationship is not evident in morning and evening-types with extreme circadian phases(50
Furthermore, behavioral factors such as a reduction in light exposure across the day could contribute to depressive symptomatology. For example, low light levels in the fall and winter have been implicated as a mechanism in seasonal affective disorder. The limited data of light exposure in adult DSPD and evening-types are inconsistent (51
). Further support for the role of exposure to circadian synchronizing agents and therefore the circadian hypothesis comes from the finding that stabilization or strengthening of social and behavioral circadian rhythms improves recovery from bipolar depression and could decrease vulnerability to the illness (53
The finding in our study that the evening chronotype, rather than DSPD per se was associated with psychiatric disorders also supports a common physiological or molecular basis for this relationship. For example, both DSPD (56
) and evening chronotype (58
) have been associated with a longer than normal circadian period. A longer circadian period can make it more difficult to entrain to a 24 hour cycle and to maintain a regular social and professional schedule. Circadian period is likely genetically determined and linked to circadian clock genes. Several studies have found alterations in circadian clock genes in those with circadian rhythm sleep disorders and extreme circadian preference (59
). Examination of molecular changes in evening-types with and without psychiatric diagnosis may be extremely useful in elucidating the role of the circadian clock in mental health.
Results of our study are limited by aspects of our methodology. The use of a convenience sample limits the generalizablility of our findings to the population as a whole. Furthermore, while the use of a DSM-IV based diagnostic classification provides information on whether individuals meet criteria for diagnoses it does not provide information on the severity of those illnesses. The sample size for our study was relatively small, yet larger than most studies of DSPD. However, a post-hoc power analysis demonstrated power >90% for more than one past diagnosis and phobia (66
), which suggests that Type II error is an unlikely explanation for our results.
In summary, results of this study support the hypothesis of a common link or genetic susceptibility between evening circadian chronotype and psychiatric disorders, highlighting the important role of circadian regulation in mental illness. Thus, it is important for clinicians to recognize that mood, anxiety and substance use disorders are common in patients with circadian rhythm misalignment, such as seen in DSPD or extreme evening-types who attempt to conform to the “usual” work day. The likely bidirectional nature of the relationship between circadian rhythms and mood regulation suggest that a multimodal approach that incorporates both circadian measures to optimize synchronization of circadian rhythms, as well as treatment of the psychiatric disorder is likely to yield the best outcomes in the management of anxiety and mood disorders.