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People have a tendency to organize their behavior into cycles of activity and rest (Moore, 1995), and arc different from one another in the way they practice this organization. What is intriguing about these cycles is that they actually represent a very familiar phenomenon to most of us. We all seem to be aware of the fact that some people are night owls, and like to stay up late at night and sleep late in the morning, while others are early birds, and prefer to go to bed at an early hour and arise with the break of dawn. In scientific language this phenomenon is called Momingness-Eveningness (M-E), and can be defined as individual differences in sleep-wake patterns, and the time of day people feel and perform best. Here, the night owls are labeled Evening types, early birds are labeled Morning types, and in between we find a majority of people called Neutral types.

Three different samples of adolescents constitute the participants of these studies. Two samples are cohorts from the same middle size Swedish community (approximately 120,000 inhabitants), who were assessed for the first time in grade 8 (in the Swedish school system, approximately 15 years of age). The third sample consists of all children and adolescents between the ages of 10 to 18 in a small Swedish community (approximately 25,000 inhabitants).

Previous research on M-E in adolescence is rather meager. In particular, there are not many published studies reporting on the development of M-E during the adolescent years. This void in the knowledge about M-E needs to be filled, in order for research to continue dealing with M-E in terms of processes and how it may be related to development. There is no way to infer the developmental imphcations M-E, if there is no knowledge about the development of M-E itself.

The objective of this section is to describe the two- and five-year stability of M-E, on the basis of the adolescents’ ratings of themselves on the CMEQ2, and parents’ ratings of their children’s M-E. How many are Evening, Neutral, and Morning types, respectively? Do they change over a period of 18 months? Do they change over a period of five years? In that case, how? Are the adolescents’ scores on the M-E questionnaire related to information about actual times of arising, bedtimes, and curfews? Are the adolescents’ scores related to M-E of parents or peers? And also, are there gender differences in M-E that have to be taken into account when bringing research further into studying other topics in relation to M-E?

According to earlier theory building, it is reasonable to assume that Evening types will display patterns of adjustment that are distinctly different from those of Morning types (e.g., Cofer et al., 1999). However, the empirical tests of this assumption have been conspicuous by their absence. Whenever individual adjustment has been investigated in relation to M-E, it has been done in studies focusing on single variables, indicating that being an Evening type is disadvantageous (e.g., Adan, 1994; Chelminski et al., 1999; Cortesi et al., 1997; Ishihara et al., 1985; Mecacci & Rocchetti, 1998). Nevertheless, it is well known that problem behaviors tend to cluster (e.g., Stattin Sc Magnusson, 1996), and such gravitation tendencies must be taken into account when assessing the magnitude of Eveningness as a risk factor in development. Hence, the present section will address the relation between M-E and different aspects of social adjustment, both as separate entities and as patterns of behavior. The general prediction is that Eveningness should be associated with negative adjustment, whereas Morningness should be related to a positive outcome.

In the repercussions of the recent surfacing of developmental issues in M-E research, discussions about how M-E can be an important factor within the parent-child relationship have been initiated (see Cofer et al., 1999). The ideas that have been put forward are essentially elaborations, extensions, and refinements of more general suggestions that time patterns of individual family members should affect the functioning of the family, as a whole (Hall, 1976; Kantor & Lehr, 1975; Moore, 1963; Osborne & Fincham, 1994; Young & Ziman, 1971). Previous research has indeed addressed these proposals, but the empirical studies have mainly focused on partner relationships (e.g., Adams & Cromwell, 1978; Hoskins, 1979, 1989; Hoskins & Halberg, 1983; Hoskins et al., 1979; Larson et al., 1991) and less on parent-child interactions (although see Cofer et al., 1999; Stone et al., 1991). Also, the single study of differences in parent-child interactions among M-E types focused specifically on conflicts (i.e., Cofer et al., 1999). This means that other indicators of family functioning have, up to now, been left out of M-E research.

If M-E can be associated with a person’s patterns of problem behavior and interactions with family members, other behaviors and interactions with other individuals could also be linked to M-E. In fact, any situation or affiliation between individuals that is in some way dependent on time limits or preferences could be expected to be related to M-E. School could be such a setting, and patterns of interaction related to M-E could be found in adolescent-teacher relationships.

Patterns of adjustment, problem gravitation, and problem aggregation are facets of comorbidity that should be addressed when discussing adolescent risk behaviors and their role in future adjustment problems (Stattin & Magnusson, 1996). Problem gravitation denotes a tendency for adjustment problems to gravitate to smaller clusters of persons with certain characteristics. Patterns of adjustment and problem aggregation deal with multiple problems within the individual. In the case of M-E, the results in the present volume have shown that Evening types tend to have adjustment difficulties in the social, family, and school arenas. This could be seen as indicative of problem gravitation, but does not really show the magnitude of that gravitation, or the patterns that the different adjustment problems form for Evening types.

The developmental theoretical framework that has been outlined by Cofer and colleagues (see Cofer et al., 1999) suggests that Eveningness is equal to risk across contexts. This negative development has been attributed to feelings of alienation from family and society that eventually cause Evening type adolescents to choose a more deviant pathway. It has also been assumed that causality more or less originates within M-E, but that has never actually been tested until now. The literature has offered an alternative explanation to the link between M-E and social adjustment, namely that of reversed causality (Stattin & Kerr, 1999). This suggestion stems from the idea that it is just as likely for an adolescent abeady in trouble to display a temporal preference similar to Eveningness, as it would be for an Evening type to develop problems. Also, as I have suggested earlier in the present volume, correlated factors could drive both a development toward Eveningness as well as toward negative development. This would make it seem as if Eveningness and the negative behavior are related, even if they are only connected by the means of the underlying factor that they have in common.

The skewed distributions of M-E types that were found in the present samples, is a skewness that should perhaps be expected, considering earlier research where a larger proportion of Evening types have been found among adolescents (e.g., Carskadon et al., 1993; Hasher et al., 1999; Ishihara et al., 1990; Nordlund & Lennem’s Junberger, 2001; Park et al., 1999; Shinkoda et al., 2000). Some argue that the distributions should be normal (e.g., Cofer et al., 1999). However, the most relevant discussion in this matter concerns the way in which a skewed distribution could have affected the results. The cutting points used here yielded significantly more Evening types adolescents than Morning types, and individuals with the most severe problem behaviors were found among the Evening types. At the same time, there were Evening types who had few or no problems at all. According to the reasoning that Eveningness is a risk, excluding misclassified, less extreme Evening type adolescents should increase the differences between Evening, Neutral, and Morning types. Thus, if the skewness had any effect on the results, it most likely weakened the conclusions about Eveningness and problem behavior. Similarly, if Momingness was protective, the protective effect should most certainly have shown up in the small group of extreme Morning types that was used here. Hence, if the skewness at this end had any effect on the results, it, too, would most probably have weakened the conclusions.