H.P.A. van Dongen

Morning-type and evening-type individuals differ in the phase position of their endogenous circadian oscillator

In dealing with inter-individual phase differences in overt circadian rhythms, it is often difficult to distinguish the impact of the endogenous circadian oscillator from that of an individuals habitual lifestyle. In an attempt to resolve this uncertainty about the role of masking influences, two groups of subjects, morning-type and evening-types, were selected and monitored during entrained, habitual sleep-wake conditions and during 24 h of controlled wakefulness in a laboratory-based constant-routine procedure. Under both conditions significant differences were observed in the circadian phases of body temperature and subjective alertness. During constant routine mean between-group differences for these two variables were 2.21 and 4.28 h, respectively. Thus, evidence is provided for the endogenous nature of morningness-eveningness.

Sleep Loss Reduces Diurnal Rhythm Amplitude of Leptin in Healthy Men

The aim of the current study was to investigate the effects of sleep loss on the diurnal rhythm of circulating leptin levels. An indwelling forearm catheter was used to sample blood at 90‐min intervals for a total of 120 h, which included 88 h of sustained sleeplessness, in 10 healthy men. The diurnal amplitude of leptin was reduced during total sleep deprivation and returned toward normal during the period of recovery sleep. This finding provides evidence that sleep influences the nocturnal leptin profile, and may have implications for the understanding of the role of sleep in metabolic regulation and the aetiologies of obesity and the night eating syndrome.

Chronic Insomnia and Daytime Functioning: An Ambulatory Assessment

In this ambulatory study, the relation between daytime functioning and chronic insomnia was investigated. The study sample consisted of 39 chronic insomniacs and 20 healthy control participants. Performance (vigilance, working memory, motor control) and well-being (concentration, fatigue, mood, sleepiness) were assessed by means of a validated test battery, and intra-individual sleep variability was taken into account. Subjective well-being was found to be compromised in insomniacs as compared to control participants, but no differences in the level of performance were found. Evening cortisol levels did not indicate increased levels of arousal in the insomniacs. Although the absence of an effect of chronic insomnia on objectively measured performance may be due to experimental or statistical factors, this study hypothesizes that the insomniacs studied in the field may have been able to exert compensatory effort to overcome their self-perceived fatigue.

Absence of Seasonal Variation in the Phase of the Endogenous Circadian Rhythm in Humans

Humans may be subject to seasonal variations, as evidenced by the existence of seasonal affective disorder (SAD) and midwinter insomnia. However, some recent studies have shown that the seasonal variation in the phase of the circadian rhythm is relatively weak in healthy humans. In the present study, evidence is found that there is no seasonal variation in the phase of the endogenous circadian rhythm at all. Body temperature, cortisol excretion, and subjective alertness of six subjects recorded under constant routine conditions showed no systematic seasonal variation in circadian phases. This finding indicates that secondary zeitgebers blocked or counterbalanced the seasonal variation in the entrainment effect of the natural photoperiod. The human being may live in an environment in which the photoperiod has lost its status of primary zeitgeber.