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L C Lack

Publications and source records attributed to L C Lack.

9 recordsLinked to original sources
PubMed2001

The short-term benefits of brief and long naps following nocturnal sleep restriction.

STUDY OBJECTIVES: The purpose was to remedy the lack of experimental studies directly comparing the effects of brief and long daytime naps following nocturnal sleep restriction. DESIGN: Twelve young adult healthy sleepers participated in a repeated measures design comparing the effects of no nap, a 10-minute nap, and a 30-minute afternoon nap in each case following a night of 4.7 hours of total sleep time. Objective and subjective alertness measures and cognitive performance measures were taken before, then 5, 35, and 60 minutes after the termination of the nap. SETTING: N/A. PARTICIPANTS: N/A. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: In the no nap condition measures showed either no change or a decreases of alertness and performance across the testing period. Following the 10-minute nap there was an immediate improvement in subjective alertness and cognitive performance which was sustained for the hour of post nap testing. Immediately following the 30 minute nap most measures of alertness and performance declined but showed some recovery by the end of testing. CONCLUSIONS: Because the delayed benefits following the 30-minute nap may be due to sleep inertia, longer post-nap testing periods should be investigated. However, we conclude that the detrimental effects of sleep restriction were more rapidly and significantly ameliorated, at least within the hour following the nap, by a 10-minute afternoon nap.

Adult↗
PubMed2001

Light emitting diodes can be used to phase delay the melatonin rhythm.

Two portable light sources, comprising light emitting diodes (LEDs) of two different wavelengths, were compared to a standard light box in suppressing and phase shifting nocturnal salivary melatonin. All light sources were equated for illuminance of 2000 lux. Sixty-six volunteers participated in the 2-day study and were randomly allocated to one of four conditions; light box, white LED, blue/green LED, or no light control group. Light was administered to the experimental groups from midnight to 02.00 hr on the first night. Half-hourly saliva samples were collected from 19.00 to 02.00 hr on night 1 and until 01.00 hr on night 2. Percent melatonin suppression on night 1 and dim light melatonin onset (DLMO) for each night were calculated. The experimental groups showed significant melatonin suppression during light stimulation, with the blue/green LEDs producing the greatest (70%) suppression. There was no significant difference between the light box at 63% and white LED at 50% suppression. Similarly, the blue/green LED had a significantly greater DLMO delay of 42 min and no difference between the light box of 23 min and the white LED of 22 min. These data suggest the portable LED light source is an effective way of delivering light to phase shift the melatonin rhythm, with the blue/green LED being the more effective of the two LEDs.

Adult↗
PubMed2001

Effect of light wavelength on suppression and phase delay of the melatonin rhythm.

Different wavelengths of light were compared for melatonin suppression and phase shifting of the salivary melatonin rhythm. The wavelengths compared were 660 nm (red), 595 nm (amber), 525 nm (green), 497 nm (blue/green), and 470 nm (blue). They were administered with light-emitting diodes equated for irradiance of 130 muW/cm2. Fifteen volunteers participated in all five wavelength conditions and a no light control condition, with each condition conducted over two consecutive evenings. Half-hourly saliva sam ples were collected from 19:00 to 02:00 on night 1 and until 01:00 on night 2. Light was administered for the experimental conditions on the first night only from midnight to 02:00. Percentage melatonin suppression on night 1 and dim light melatonin onset (DLMO) for each night were calculated. The shorter wavelengths of 470, 497, and 525 nm showed the greatest melatonin suppression, 65% to 81%. The shorter wavelengths also showed the greatest DLMO delay on night 2, ranging from 27 to 36 min. The results were consistent with the involvement of a scotopic mechanism in the regulation of circadian phase.

Adult↗
PubMed1996

The rhythms of human sleep propensity and core body temperature.

Evidence from human free-running studies has suggested a close relationship between the timing of the circadian rhythm of core body temperature and the rhythm of sleep propensity. However, this relationship may be questioned by variations of sleep and wakeful activity which could have masked the endogenous temperature rhythm. A constant routine was used here to 'unmask' the endogenous temperature rhythm in addition to frequent sleep trials across a 24-h period to confirm the relationship between temperature and sleep propensity rhythms. Of the 14 healthy, good sleeping subjects 13 had significant 24-h cosine rhythms of sleep propensity. Eight of these also had a significant 12-h cosine rhythm. The eight subjects with both 24-h and 12-h rhythms showed a minor peak of sleep propensity in the early afternoon followed by a though in the early evening (20.00 hours). Sleep propensity then rose rapidly at about midnight to a major peak in the early morning. This was followed by a second trough of sleep propensity in the late morning. The average times of the sleep propensity phases relative to the circadian temperature rhythm were very similar to the earlier free-running studies. Furthermore, the times of the sleep propensity phases were highly correlated with the body temperature minimum. These results suggested the possibility that a common oscillator determines the timing of both the body temperature rhythm and the phases of the sleep propensity rhythm.

Adult↗
PubMed1996

Circadian rhythms of early morning awakening insomniacs.

It has been suggested that two types of insomnia, sleep onset insomnia and early morning awakening insomnia, may be caused by delays and advances respectively of circadian rhythms. Evidence supports the circadian rhythm phase delay of sleep onset insomniacs. The present study investigated the phase timing of circadian rhythms of early morning awakening insomniacs compared with a group of age matched good sleepers. A 24-h bed rest laboratory session was used to evaluate the endogenous core body temperature and urinary melatonin rhythms. Objective and subjective sleepiness were also measured every 30 min across the session with 10 min multiple sleep latency tests and Stanford Sleepiness Scale. Maximum and minimum phases of each individual's rhythm were identified using two-component cosine curve fitting. Compared with the good sleepers, the insomniacs had significant phase advances of 2-4 h for the temperature and melatonin rhythms. However, the 0-4 h advances of the sleepiness rhythms were not significant. This latter unexpected result was explained on the basis of variability of sleepiness measures. It was suggested that early morning awakening insomnia arises from phase advanced circadian rhythms which evoke early arousal's from sleep.

Adult↗
PubMed1984

The effect of sleep before or after learning on memory.

Early studies in which it was found that learning followed by sleep was better remembered than learning followed by wakefulness were interpreted as giving support for the Interference Theory of Forgetting. More recent studies have shown better retention over the first half of the night's sleep (slow-wave sleep) than over the second half (REM sleep), and conclusions have been drawn that a Decay Theory of Forgetting is more strongly supported. Those studies, however, confounded the type of sleep following learning with sleep prior to learning. When prior sleep was controlled in the present study, there was no support for a first half-night sleep benefit, and, contrary to Decay Theory, there was a second half-night benefit for high imagery material. The strong detrimental effect of sleep prior to learning was inconsistent with the Interference Theory of Forgetting and suggested, instead, the importance of the consolidation process for long-term memory.

Adult↗