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The effect of twilight transitions on entrainment of C57BL/6JOlaHsd mice (Mus musculus) was studied using light-dark cycles of different photoperiods (6, 12, and 18 h) and twilight transitions of different durations (0, 1, and 2 h). Phase angle differences of the onset, center of gravity, and offset of activity, activity duration (alpha), as well as free-running period (tau) in continuous darkness were analyzed. The main finding was that for all conditions the onset of activity was close to dusk or lights-off except for the short photoperiod with 2 h of twilight where activity onset was on average 5.3 (SEM 1.07) h after lights-off. This finding contrasts with the results of Boulos and Macchi for Syrian hamsters where a 5.9-h earlier activity onset was observed when similar photoperiod and twilight conditions are compared with a rectangular LD cycle. The authors suggest the opposite effects of 2 h of twilight in the 2 species may be related to their different free-running periods under DD conditions following entrainment to short photoperiod with 2-h twilight conditions. Since the authors observed larger variation in phase angle of entrainment in longer twilight conditions, twilight does not necessarily form a stronger zeitgeber.

Phase shifting of circadian systems by light has been attributed both to parametric effects on angular velocity elicited by a tonic response to the luminance level and to nonparametric instantaneous shifts induced by a phasic response to the dark-light (D>L) and light-dark (L>D) transitions. Claims of nonparametric responses are partly based on "step-PRCs," that is, phase response curves derived from such transitions. Step-PRCs in nocturnal mammals show mostly delays after lights-on and advances after lights-off, and therefore appear incompatible with phase delays generated by light around dusk and advances by light around dawn. We have pursued this paradox with 2 experimental protocols in mice. We first use the classic step-PRC protocol on wheel running activity, using the center of gravity as a phase marker to minimize the masking effects of light. The experiment was done for 3 different light intensities (1, 10, and 100 lux). D>L transitions evoke mostly delays and L>D transitions show no clear tendency to either delay or advance. Overall there is little or no circadian modulation. A 2nd protocol aimed to avoid the problem of masking by assessing phase before and after the light stimuli, both in DD. Light stimuli consisted of either a slow light intensity increase over 48 h followed by abruptly switching off the light, or an abrupt switch on followed by a slow decrease toward total darkness during 48 h. If the abrupt transitions were responsible for phase shifting, we expected large differences between the 2 stimuli. Both light stimuli yielded similar PRCs characterized by delays only with circadian modulation. The results can be adequately explained by a model in which all PRCs evoked by steps result in fact from tonic responses to the light following a step-up or preceding a step-down. In this model only the response reduction of tonic velocity change after the 1st hour is taken into account. The data obtained in both experiments are thus compatible with tonic velocity responses. Contrary to standard interpretation of step-PRCs, nonparametric responses to the transitions are unlikely since they would predict delays in response to lights-off, advances in response to lights-on, while the opposite was found. Although such responses cannot be fully excluded, parsimony does not require invocation of a role for transitions, since all the data can readily be explained by tonic velocity (parametric) effects, which must exist because of the dependence of tau on light intensity.

The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber signal depends on (a) characteristics of the entraining signal and (b) response characteristics and intrinsic stability of the pacemaker itself. Position of the sun, weather conditions, shades, and behavioral variations (eye closure, burrowing) all modulate the light signal reaching the pacemaker. A simple model of a circadian pacemaker allows researchers to explore the impact of these factors on pacemaker accuracy. Accuracy is operationally defined as the reciprocal value of the day-to-day standard deviation of the clock times at which a reference phase (0) is reached. For the purpose of this exploration, the authors used a model pacemaker characterized solely by its momentary phase and momentary velocity. The average velocity determines the time needed to complete one pacemaker cycle and, therefore, is inversely proportional to pacemaker period. The model pacemaker responds to light by shifting phase and/or changing its velocity. The authors assumed further that phase and velocity show small random fluctuations and that the velocity is subject to aftereffects. Aftereffects were incorporated mathematically in a term allowing period to contract exponentially to a stable steady-state value, with a time constant of 69 d in the absence of light. The simulations demonstrate that a pacemaker reaches highest accuracy when it responds to light by simultaneous phase shifts and changes of its velocity. Phase delays need to coincide with slowing down and advances with speeding up; otherwise, no synchronization to the zeitgeber occurs. At maximal accuracy, the changes in velocity are such that the average period of the pacemaker under entrained conditions equals 24 h. The results suggest that during entrainment, the pacemaker adjusts its period to 24 h, after which daily phase shifts to compensate for differences between the periods of the zeitgeber and the clock are no longer necessary. On average, phase shifts compensate for maladjustments of phase and velocity changes compensate for maladjustments of period.

Observational data collected in the field and in enclosures show that diurnal, burrow-dwelling European ground squirrels (Spermophilus citellus) never were above ground during twilight at dawn or at dusk. The animals emerged on average 4.02 h (SD = 0.45) after civil twilight at dawn and retreated in their burrows on average 2.87 h (SD = 0.47) before civil twilight at dusk. Daily patterns of light perceived by these burrowing mammals were measured with light-sensitive radio collar transmitters in an enclosure (the Netherlands) and in the field (Hungary). The observational data are corroborated by the telemetry data, which show clear daily patterns of timing of light perception including light perceived from the burrow entrances. The first light was observed by the animals on average 3.54 h (enclosure, SD = 0.45) and 3.60 h (field, SD = 0.31) after civil twilight at dawn, whereas the final observed light was on average 3.04 h (enclosure, SD = 0.64) and 2.02 h (field, SD = 0.72) before civil twilight at dusk. Thus, the animals do not perceive the rapid natural light-dark (LD) transitions that occur at civil twilight. Instead, they generate their own pattern of exposure to light within the natural LD cycle. The classical phase response model for entrainment by light or dark pulses cannot explain how the circadian system of this species remains entrained to the external, natural LD cycle while the major LD transitions are created by its own behavior.

Light-induced improvements in alertness are more prominent during nighttime than during the day, suggesting that alerting effects of light may depend on internal clock time or wake duration. Relative contributions of both factors can be quantified using a forced desynchrony (FD) designs. FD designs have only been conducted under dim light conditions (<10 lux) since light above this amount can induce non-uniform phase progression of the circadian pacemaker (also called relative coordination). This complicates the mathematical separation of circadian clock phase from homeostatic sleep pressure effects. Here we investigate alerting effects of light in a novel 4 × 18 h FD protocol (5 h sleep, 13 h wake) under dim (6 lux) and bright light (1300 lux) conditions. Hourly saliva samples (melatonin and cortisol assessment) and 2-hourly test sessions were used to assess effects of bright light on subjective and objective alertness (electroencephalography and performance). Results reveal (1) stable free-running cortisol rhythms with uniform phase progression under both light conditions, suggesting that FD designs can be conducted under bright light conditions (1300 lux), (2) subjective alerting effects of light depend on elapsed time awake but not circadian clock phase, while (3) light consistently improves objective alertness independent of time awake or circadian clock phase. Reconstructing the daily time course by combining circadian clock phase and wake duration effects indicates that performance is improved during daytime, while subjective alertness remains unchanged. This suggests that high-intensity indoor lighting during the regular day might be beneficial for mental performance, even though this may not be perceived as such. [ABSTRACT FROM AUTHOR]

Human thermoregulation is strictly regulated by the preoptic area of the hypothalamus, which is directly influenced by the suprachiasmatic nucleus (SCN). The main input pathway of the SCN is light. Here, thermoregulatory effects of light were assessed in humans in a forced desynchrony (FD) design. The FD experiment was performed in dim light (DL, 6 lux) and bright white light (BL, 1300 lux) in 8 men in a semi-randomized within-subject design. A 4 × 18 h FD protocol (5 h sleep, 13 h wake) was applied, with continuous core body temperature (CBT) and skin temperature measurements at the forehead, clavicles, navel, palms, foot soles and toes. Skin temperature parameters indicated sleep-wake modulations as well as internal clock variations. All distal skin temperature parameters increased during sleep, when CBT decreased. Light significantly affected temperature levels during the wake phase, with decreased temperature measured at the forehead and toes and increased navel and clavicular skin temperatures. These effects persisted when the lights were turned off for sleep. Circadian amplitude of CBT and all skin temperature parameters decreased significantly during BL exposure. Circadian proximal skin temperatures cycled in phase with CBT, while distal skin temperatures cycled in anti-phase, confirming the idea that distal skin regions reflect heat dissipation and proximal regions approximate CBT. In general, we find that increased light intensity exposure may have decreased heat loss in humans, especially at times when the circadian system promotes sleep. [ABSTRACT FROM AUTHOR]

Under real-life conditions, increased light exposure during wakefulness seems associated with improved sleep quality, quantified as reduced time awake during bed time, increased time spent in non-rapid eye movement (NREM) sleep, or increased power of the electroencephalogram delta band (0.5-4 Hz). The causality of these important relationships and their dependency on circadian phase and/or time awake has not been studied in depth. To disentangle possible circadian and homeostatic interactions, we employed a forced desynchrony protocol under dim light (6 lux) and under bright light (1300 lux) during wakefulness. Our protocol consisted of a fast cycling sleep-wake schedule (13 h wakefulness—5 h sleep; 4 cycles), followed by 3 h recovery sleep in a within-subject cross-over design. Individuals (8 men) were equipped with 10 polysomnography electrodes. Subjective sleep quality was measured immediately after wakening with a questionnaire. Results indicated that circadian variation in delta power was only detected under dim light. Circadian variation in time in rapid eye movement (REM) sleep and wakefulness were uninfluenced by light. Prior light exposure increased accumulation of delta power and time in NREM sleep, while it decreased wakefulness, especially during the circadian wake phase (biological day). Subjective sleep quality scores showed that participants rated their sleep quality better after bright light exposure while sleeping when the circadian system promoted wakefulness. These results suggest that high environmental light intensity either increases sleep pressure buildup during wakefulness or prevents the occurrence of micro-sleep, leading to improved quality of subsequent sleep. [ABSTRACT FROM AUTHOR]