Influence of nocturnal and daytime sleep on initial orthostatic hypotension.

PURPOSE: The incidence of vasovagal syncope is more common in the morning. Previous researchers have reported negligible diurnal variation in the physiological responses associated with initial orthostatic hypotension (IOH). Nevertheless, physical activity and sleep prior to morning and afternoon test times have not been controlled and may influence the findings. We designed a semi-constant routine protocol to examine diurnal variation in cardiorespiratory and cerebrovascular responses to active standing. METHODS: At 06:00 and 16:00 hours, nine males (27 ± 9 years) completed an upright-stand protocol. Altimetry-measured sleep durations were 3.3 ± 0.4 and 3.2 ± 0.6 h immediately prior to the morning and afternoon test times. Continuous beat-to-beat measurements of middle cerebral artery velocity (MCAv), mean arterial blood pressure (MAP), heart rate (HR), and end-tidal carbon dioxide were obtained. Intestinal body temperature and salivary melatonin concentrations were also measured. RESULTS: Compared with the afternoon, resting HR and body temperature were 4 ± 2 beats min(-1) and 0.45 ± 0.2 °C lower, respectively, whereas melatonin concentration was 28.7 ± 3.2 pg ml(-1) higher in the morning (P ≤ 0.02). Although all individuals experienced IOH at both times of the day, the initial decline in MAP during standing was 13 ± 4 mmHg greater in the afternoon (P = 0.01). Nevertheless, the decline in MCAv was comparable at both times of day (mean difference: 2 ± 3 cm s(-1); P = 0.5). CONCLUSION: These findings indicate that a bout of sleep in the afternoon in healthy young individuals results in greater IOH that is compensated for by effective cerebral blood flow regulation.

A meta-analytic approach to quantify the dose-response relationship between melatonin and core temperature.

A melatonin-mediated reduction in body temperature could be useful as a "pre-cooling" intervention for athletes, as long as the melatonin dose is optimised so that substantial soporific effects are not induced. However, the melatonin-temperature dose-response relationship is unclear in humans. Individual studies have involved small samples of different sexes and temperature measurement sites. Therefore, we meta-analysed the effects of exogenous melatonin on body core temperature to quantify the dose-response relationship and to explore the influence of moderating variables such as sex and measurement site. Following a literature search, we meta-analysed 30 data-sets involving 193 participants and 405 ingestions of melatonin. The outcome was the mean difference (95 % confidence limits) in core temperature between the melatonin and placebo-controlled conditions in each study, weighted by the reciprocal of each standard error of the difference. The mean (95 % confidence interval) pooled reduction in core temperature was found to be 0.21 °C (0.18-0.24 °C). The dose-response relationship was found to be logarithmic (P < 0.0001). Doses of 0-5 mg reduced temperature by ~0.00-0.22 °C. Any further reductions in temperature were negligible with doses >5 mg. The pooled mean reduction was 0.13 °C (0.05-0.20 °C) for oral temperature vs 0.26 °C (0.20-0.32 °C) for tympanic and 0.22 °C (0.19-0.25 °C) for rectal temperature. In conclusion, our meta-regression revealed a logarithmic dose-response relationship between melatonin and its temperature lowering effects. A 5-mg dose of melatonin lowered core temperature by ~0.2 °C. Higher doses do not substantially increase this hypothermic effect and may induce greater soporific effects.

Vitamin E supplementation does not alter physiological performance at fixed blood lactate concentrations in trained runners.

AIM: The aim of the study was to determine the direction of change in performance variables at fixed blood lactate concentrations following vitamin E (VE) supplementation. METHODS: In a paired-matched design twelve (male: N.=8; female: N.=4) trained runners were allocated to a VE (N.=6; 268 mg·d⁻¹) or placebo (N.=6; glucose: 30 mg·d⁻¹) group for 35 days. Participants completed a discontinuous incremental exercise test, pre and post supplementation, to determine peak oxygen uptake (VO2peak) running velocity and percentage of peak oxygen uptake (%(VO2peak) at the lactate threshold (TLAC) and the onset of blood lactate accumulation (OBLA). Participants maintained a standardised training regime throughout the supplementation period. RESULTS: VE supplementation failed to significantly enhance velocity at TLAC (P=0.91) and OBLA (P=0.22) compared to a placebo treatment. Analogously, VE did not significantly enhance %(VO2peak) at TLAC (P=0.85) and OBLA (P=0.71) compared to a placebo treatment. Whilst VE supplementation did not enhance performance it did not impair performance compared to a placebo. Training significantly enhanced velocity at TLAC (P=0.00) and OBLA (P=0.05). No training-induced improvements in %VO2peak at TLAC (P=0.06) and OBLA (P=0.40) were observed. CONCLUSION: Daily VE supplementation for 35 days does not enhance or impair physiological performance at fixed blood lactate concentrations. Long-term VE supplementation for the purposes of performance enhancement is not recommended.

Diurnal variation in the salivary melatonin responses to exercise: relation to exercise-mediated tachycardia.

Salivary melatonin concentration is an established marker of human circadian rhythmicity. It is thought that melatonin is relatively robust to the masking effects of exercise. Nevertheless, the extent and even the direction of exercise-related change is unclear, possibly due to between-study differences in the time of day exercise is completed. Therefore, we aimed to compare melatonin responses between morning and afternoon exercise, and explore the relationships between exercise-related changes in melatonin and heart rate. At 08:00 and 17:00 hours, seven male subjects (mean ± SD age, 27 ± 5 years) completed 30 min of cycling at 70% peak oxygen uptake followed by 30 min of rest. Light intensity was maintained at ~150 lx. Salivary melatonin (ELISA) and heart rate were measured at baseline, 15 min during exercise, immediately post-exercise and following 30 min recovery. Melatonin was ≈15 pg ml(-1) higher in the morning trials compared with the afternoon (P = 0.030). The exercise-related increase in melatonin was more pronounced (P = 0.024) in the morning (11.1 ± 8.7 pg ml(-1)) than in the afternoon (5.1 ± 5.7 pg ml(-1)). The slope of the heart rate-melatonin relationship was significantly (P = 0.020) steeper in the morning (0.12 pg ml(-1) beats(-1 )min(-1)) than in the afternoon (0.03 pg ml(-1) beats(-1 )min(-1)). In conclusion, we report for the first time that the masking effect of moderate-intensity exercise on melatonin is approximately twice as high in the morning than the afternoon. The much steeper relationship between heart rate and melatonin changes in the morning raises the possibility that time of day alters the relationships between exercise-mediated sympathetic nervous activity and melatonin secretion.

An investigation into the physical determinants of change of direction speed.

AIM: Change of direction speed (CODS) is an important attribute for many sports and is believed to be influenced by a variety of physical factors. However, there is a lack of consensus as to which physical attributes relate to CODS. The aim of this study was to examine the relationship of several physical attributes to CODS. METHODS: Thirty-eight subjects (mean+/-SD: age, 21.5+/-3.8 years; height, 1.77+/-0.07 m; mass, 77.5+/-13.9 kg) undertook tests of speed, CODS, strength and power. Running speed was assessed via a 25 m sprint with split times taken at 5, 20 and 25 m. CODS was assessed by a 505-test, which involves measuring the time to complete a 5 m out and back course. The strength and power tests included unilateral isokinetic concentric and eccentric knee extensor and flexor strength at 60 degrees /s and bilateral leg press, countermovement and drop jumps. Pearson's product moment correlation and co-efficients of determination were used to explore relationships amongst all variables. Multiple regression was used to determine the combined effects of significantly correlated variables on CODS. RESULTS: Stepwise multiple regression revealed that running speed explained 58% of the variance in CODS (F(1,33)=45.796, P<0.001) with the addition of eccentric knee flexor strength raising the value to 67% (F(1,32)=8.781, P=0.006). CONCLUSIONS: The results suggest that for basic improvements in CODS, athletes should seek to maximise their sprinting ability and enhance their eccentric knee flexor strength to allow effective neuromuscular control of the contact phase of the CODS task.