How many diurnal types are there? A search for two further “bird species”

Morning and evening types (“larks” and “owls”) are most alert in the morning and in the evening, respectively. Because they are also characterized by preference for early awakening-early bedtime and late awakening-late bedtime, respectively, two questions arise: Is it possible to distinguish two additional types preferring early awakening-late bedtime and late awakening-early bedtime? If yes, are they similar to the types of habitual short and long sleepers? One hundred and thirty healthy participants of sleep deprivation experiments were subdivided into four (2 × 2) types depending upon self-assessed preferences for morning and evening earliness/lateness. The differences between these types in self-assessed morning/evening earliness/lateness were associated with the differences in levels of morning/evening-early night sleepiness. However, self-reports on their pre-experimental wakeups/bedtimes showed that the two additional types were not identical to the types of short and long sleepers. It seems that the four-type classification of morning/evening preference represents pairwise combinations of low/high levels of waking ability during the morning/evening-early night hours, and that such variation in waking ability is irrelevant to individual differences in sleep ability, i.e., variation in sleep need, sleep capacity, sleep quality, napping propensity, etc.