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Keywords:

  • neuroendocrine;
  • obesity;
  • sleep restriction

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

This study examined the effects of two nights of sleep restriction on neuroendocrine hormones (i.e. peptide YY [PYY], ghrelin, glucagon-like peptide-1, adiponectin, and leptin) involved in regulating body weight. Ten healthy male adults aged 18 to 23 years were subjected to two consecutive nights of sleep restriction. Compared to a night of normal sleep, sleep restriction was associated with a significant reduction in PYY levels (P= 0.047) and a significant reduction in satiety levels (P= 0.033). These results suggest that sleep restriction alters the hormonal regulation of appetite in a manner predictive of increased energy intake.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Chronic sleep restriction (i.e. sleep durations less than 7 h a night) has recently been identified as a possible risk factor for obesity.1,2 It has subsequently been suggested that chronic sleep restriction could be targeted alongside other interventions to aid the treatment and prevention of obesity. Some recent studies have indicated that chronic sleep restriction may contribute to the development of obesity by altering the hormonal regulation of body weight. For example, in humans short-term sleep restriction over two to six nights has been associated with increases in the hormone ghrelin and reductions in the hormone leptin.1,3,4 Ghrelin, which is released from the gastrointestinal tract, acts on hypothalamic receptors to increase food intake, whereas leptin is released from adipose tissue and acts on the hypothalamus to reduce food intake and increase energy expenditure.5 Sleep restriction may therefore contribute to obesity by altering the neuroendocrine regulation of body weight.

These results provide some indication of the potential mechanisms linking chronic sleep restriction to obesity, but more research is needed. Consequently, the aim of the present study was to further investigate the effects of sleep restriction on neuroendocrine functioning in humans. Consistent with previous studies, this involved examining the effects of sleep restriction on hunger and appetite, and the hormones ghrelin and leptin.3,4 However, we also examined the effects of sleep restriction on other important hormones involved in body-weight regulation that have not yet been examined in this context. These included the gastrointestinal hormones peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) which act to inhibit food intake, and the adipose tissue hormone adiponectin which acts to reduce food intake and increase energy expenditure.5,6

METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Ten healthy non-obese males aged 19 to 23 years (mean, 20.40; SD, 1.55), with self-reported habitual sleep durations between 7 and 9 h a night, participated in this study. None of the participants reported medical conditions such as diabetes, hypertension, depression, or sleep apnea that could have affected the results.

Adiponectin, ghrelin, PYY, leptin, and GLP-1 were assayed in duplicate using Millipore Lincoplex kits and a Luminex IS100 instrument (Lincoplex, Billerica, MA, USA). The assay kit for ghrelin, PYY, leptin, and GLP-1 had an intra-assay variation of <11%, and sensitivities of 157.2 pg/ml (leptin), 5.2 pg/ml (GLP-1), 1.8 pg/ml (ghrelin), and 8.4 pg/ml (PYY). The assay kit for adiponectin had an intra-assay variation of 1.4–7.9%, and a sensitivity of 80.3 pg/ml.

Changes in ratings of hunger and satiety levels were examined using visual analog scales (VAS), where each item consisted of a 10-cm line with two opposing statements at either end (e.g. “I am not hungry at all”/“I have never been hungrier”).7 Participants responded to each item by marking a point on the line that best reflected how they felt at that particular time.

The study was conducted over four consecutive nights, consisting of a baseline night (8 h of sleep), two nights of sleep restriction (5 h of sleep each night), and a night of extended sleep (8 to 10 h of sleep). Based on previous research, two nights of 5 h of sleep was deemed sufficient to observe the physiological effects of sleep restriction.3,4 Participants reported to the laboratory each night at 19.30 hours to complete the VAS and were served a standardised evening meal (1546–1992 KJ). Participants slept in the laboratory for a maximum of 8 h (22.30–06.30 hours) on the baseline night and a maximum of 5 h (01.30–06.30 hours) on the sleep restriction nights. Supervisors monitored participants and ensured they adhered to the study protocol. On the extended sleep night, participants slept at home for 8 to 10 h to ensure they were fully rested. Participants had blood drawn each morning upon waking (except after the first night of sleep restriction), completed the VAS, and were provided with a standardized breakfast (approximately 670 KJ). Participants left the laboratory during the day to engage in normal daily activities, but kept their physical activity levels and energy intake patterns constant. The protocol for this study was approved by the Human Research Ethics Committee at the University of Wollongong; informed consent was obtained from all participants.

Data were analyzed using permutation tests, which are recommended for small sample sizes (n≤ 10), particularly when normality is violated.8 Matched pairs t-tests using permutation methods examined for differences in hormone levels and appetite ratings between the conditions. Since ghrelin and PYY exert opposing effects on appetite,5 a ratio of PYY to ghrelin was calculated by dividing PYY by ghrelin; we included this ratio in the analyses. Correlations were also performed to examine for possible associations between changes in hunger or satiety and changes in hormone levels across conditions. All statistical tests were two-tailed and P-values < 0.05 were considered statistically significant.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Consistent with previous studies,3,4 the most striking differences in hormone levels and satiety levels were observed between the extended sleep and sleep restriction conditions (Table 1). This is probably because participants were fully rested following the extended sleep condition whereas they may have experienced very mild sleep restriction at baseline. Therefore, for the purposes of this study, we present the results of the comparison between the extended sleep and sleep restriction conditions.

Table 1.  Comparison of mean hormone levels and hunger and appetite ratings between the sleep restriction and extended sleep conditions
 BaselineSleep restrictionExtended sleepP-values*
  • *

    P-values are for the comparisons between the sleep restriction and extended sleep conditions. SD are presented in parentheses. GLP-1, glucagon-like peptide 1; PYY, peptide YY.

Hours of sleep7.62 (0.40)4.90 (0.26)9.06 (1.74) 
Hunger (cm)5.88 (1.83)7.12 (1.29)7.15 (1.04)0.885
Satiety (cm)2.44 (1.87)1.72 (1.22)2.57 (1.39)0.033
Adiponectin (pg/ml)10.93 (2.13)10.60 (2.11)10.59 (2.15)0.983
Ghrelin (pg/ml)12.02 (6.33)13.73 (6.92)12.11 (6.52)0.340
GLP-1 (pg/ml)43.91 (32.22)46.52 (35.81)46.21 (33.69)0.919
Leptin (ng/ml)3.27 (2.59)3.84 (2.86)3.55 (2.72)0.200
PYY (pg/ml)77.75 (40.92)78.40 (42.77)89.80 (54.35)0.047
PYY to ghrelin ratio (pg/ml)7.95 (4.24)6.96 (3.70)9.96 (7.14)0.057

PYY levels were significantly lower with sleep restriction compared to extended sleep (t9= 2.215, P= 0.047) and the PYY ghrelin ratio was also lower, but the effect did not reach significance (t9= 2.162, P= 0.057). Participants reported significantly lower satiety levels at sleep restriction compared to extended sleep (t9= 2.293, P= 0.033), which were correlated with the changes in the PYY to ghrelin ratio (r7= 0.585, P= 0.095) and PYY (r7= .571, P= 0.113). Although these correlations did not reach statistical significance, the observed effect sizes are moderate and indicate that the changes in PYY and the PYY to ghrelin ratio explained 34.2% and 32.6% of the variance in satiety respectively.

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

The present results indicate that compared to a fully rested condition, two nights of sleep restriction was associated with a significant reduction in PYY levels and a near significant reduction in the PYY to ghrelin ratio. These results are important because PYY is released in response to ingested nutrients and acts on hypothalamic neurons to inhibit further food intake.6 Since reduced levels of PYY are associated with increased food intake,6 the present results suggest that sleep restriction could potentially promote weight gain by inhibiting the release of PYY. This conclusion is supported by the significant reduction in satiety levels observed with sleep restriction, which corresponded (albeit not significantly) with the changes in PYY and the PYY to ghrelin ratio.

Some recent studies have found that two to six nights of sleep restriction alters neuroendocrine hormones such as leptin and ghrelin a manner predictive of weight gain.3,4 The present results add support to these studies by identifying PYY as a further possible mechanism linking sleep restriction to obesity. These findings are novel because, to the best of our knowledge, the effects of sleep restriction on PYY have not been previously examined in humans. The precise mechanisms through which sleep restriction alters PYY levels are unclear but may involve pathways in the brainstem, amygdala, or the hypothalamus.

The present study offers further insight into the potential mechanisms linking chronic sleep restriction to obesity and the results warrant further investigation. This is especially important given that the present study was limited by a small sample size, and consequently low statistical power, which may explain why significant changes in other hormones (e.g. leptin, ghrelin, and adiponectin) were not observed. The study was also limited by a lack of an objective measure of sleep (polysomnography or actigraphy), which could have been used to explore the possible associations between altered hormone levels and disruptions in sleep stages as well as sleep fragmentation. It is also possible that the observed effects of two nights of sleep restriction may not correspond with prolonged or chronic sleep restriction in free, living humans. As a consequence, we recommend that long-term prospective studies of free, living humans examine the long-term impact of sleep restriction on neuroendocrine hormones (e.g. PYY, leptin, ghrelin) along with measures of energy intake and energy expenditure. This research will be important in clarifying the nature of the relationship between chronic sleep restriction and obesity, and may have implications for combating the present obesity epidemic.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES
  • 1
    Knutson KL, Van Cauter E. Associations between sleep loss and increased risk of obesity and diabetes. Ann. N. Y. Acad. Sci. 2008; 1129: 287304.
  • 2
    Magee CA, Iverson DC, Huang XF et al. A link between chronic sleep restriction and obesity: Methodological considerations. Public Health 2008; 122: 137381.
  • 3
    Spiegel K, Leproult R, L'Hermite-Balerioux M et al. Leptin levels are dependent on sleep deprivation: relationships with sympathovagal balance, carbohydrate regulation, cortisol, and thyrotropin. J. Clin. Endocrinol. Metab. 2004; 89: 576271.
  • 4
    Spiegel K, Tasali E, Penev P et al. Brief Communication: Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Ann. Intern. Med. 2004; 141: 84650.
  • 5
    Havel PJ. Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis. Exp. Biol. Med. 2001; 226: 96377.
  • 6
    Batterham RL, Cowley MA, Small CJ et al. Gut hormone PYY3-36 physiologically inhibits food intake. Nature 2002; 418: 6504.
  • 7
    Stubbs RJ, Hughes DA, Johnstone AM et al. The use of visual analogue scales to assess motivation to eat in human subjects: a review of their reliability and validity with an evaluation of a new hand-held computerized system for temporal tracking of appetite ratings. Br. J. Nutr. 2000; 84: 40515.
  • 8
    Todman JB, Dugard P. Single-Case and Small-n Experimental Designs: A Practical Guide to Randomization Tests. Lawrence Erlbaum: Mahwah, NJ, 2001.