In Tropical climes there are certain times of day
When all the citizens retire to take their clothes off and perspire
It's one of those rules the greatest fools obey
Because the Sun is far too sultry and one must avoid its ultra-violet rays
The natives grieve when the White Men leave their huts
Because they're obviously… definitely…Nuts!
Mad dogs and Englishmen go out in the midday sun
The Japanese don't care to, the Chinese wouldn't dare to
Hindus and Argentines sleep firmly from twelve till one
But Englishmen detest-a siesta
Noel Coward1924
THE MIDDAY SUN, IN AREAS WHERE IT PREVAILS, HAD ALWAYS DICTATED TO STOP STRENUOUS PHYSICAL ACTIVITY IN FAVOR OF A NAP, UNTIL THE SUN, as well as the accompanying high temperature, allowed for resumption of activity that had been initiated by sun rise. Noel Coward from an imperial/colonial perspective mocks the Englishmen who failed to recognize this reality. Nevertheless from this and perhaps additional perspectives westerners tend to look down on the practice of the midday nap or the siesta (in Spanish, derived from the Latin term for the sixth hour from sunrise), as an expression of inefficiency, and/or lack of motivation. Indeed it so happens, that this practice is common, in the Mediterranean area, India, Southeast Asia, and South America, where temperatures are higher, and where nations are less affluent.
In more northern or more southern countries, people may take a midday nap for different reasons: fatigue or somnolence in the former and a lifelong cultural habit in the latter. There is firm evidence that sleep latency is shorter in early afternoon and that is it is easier to fall asleep at those hours.1 Research (mostly carried out in more northern affluent countries) on the medical and physiologic significance of napping has therefore focused on it as a manifestation of fatigue, exploring the positive aspects of naps relative to modern hectic life style and diminishing sleep duration.2
For reasons as yet unclear, too long a sleep, sleep debt, and perhaps compensatory daytime napping in western countries have been associated in some studies with higher prevalence of diabetes mellitus.3,4 It is against this background that Lam and colleagues5 report in this issue of SLEEP on the effects of daytime napping in 20,431 members of the Guangzhou Health and Happiness Association for the Respectable Elders, in southern China, where daytime napping is a long respected practice. They had some interesting findings: Regular napping was associated with well ascertained diabetes mellitus. Moreover, as in studies in the Mediterranean area,6 napping was associated with better nocturnal sleep and a higher level of physical activity. As physical activity is a major factor known to prevent development of diabetes,7 this finding suggests that the nap diabetogenic stimulus (if validated) may be a strong one. The association with physical activity and additional subgroup analyses (performed in large numbers due to the sample size, and the vastness of the collected information) reduced the likelihood that napping reflects some sort of frailty or familial diabetes risk. Thus Lam et al. have effectively minimized reverse causation bias. The consistency of the association in those with previously diagnosed, as well as in those undiagnosed before the study procedures, and the association of napping with fasting glucose intolerance, strengthen the robust association of the nap with diabetes mellitus. In their study, daytime nap was also associated with other components of the metabolic syndrome (a prediabetic state): hypertension, hypertriglyceridemia, and fasting glucose intolerance.5
What is it about daytime napping that may predispose to glucose intolerance and diabetes mellitus, which on the surface seems to be somewhat counterintuitive? The possibilities of frailty, familial predisposition, sedentary lifestyle, obesity, and sleep debt apparently have been eliminated as factors in this report. A potential association with obstructive sleep apnea8 could not be ruled out, but absence of association with waist circumference and snoring, as well as the higher level of physical activity, do not support it.
The authors suggest that awakening in the afternoon in addition to the morning, may activate the sympathetic nervous system and the renin-angiotensin system that is linked to it, more than in those who do not nap, and adversely affect carbohydrate metabolism.5 Indeed heart rate, a crude surrogate for sympathetic activity was found to increase after daytime napping9 although less than in the morning.10 An additional possibility could be entrainment of the suprachiasmatic nucleus (SCN).11 It is likely that metabolic and circadian regulation developed to cope with adverse conditions when our prehistoric ancestors' sleep patterns were more closely synchronized to the sunrise and darkness. They consumed seasonally available food and were exposed to imminent famine dangers. A thrifty genotype is speculated to have evolved, so that during seasons of food abundance induction of reduced insulin sensitivity, hyperinsulinemia, and glucose intolerance would promote deposition of fat to be stored for periods when food is sparse. A biological clock centrally located at SCN evolved to synchronize activity and the rest of clocks located peripherally to the circadian and annual cycles, using hormones and neural networks that generate and control autonomic rhythms. The SCN requires repeated external metabolic as well as light, sleep, activity, and nutrient intake signals.11 Genetic disruption or interference with such clocks was found to induce increase feeding, obesity and blood pressure in various experimental models.11,12 Could life long napping cause such an acquired disturbance? Only future research focused in such a direction could resolve the issue.
A recent study in the US (where napping may be more common then considered by many, yet less frequent and less acceptable by the society at large) also found an association between napping and diabetes in older adults, especially longer naps.13 The study had used a large number of subjects, but it did not collect as many pieces of data about participants and could not perform as detailed a set of analyses as Lam and colleagues.5 Yet the association of napping and diabetes reported in large yet different populations3,4,13 suggests that it may be the nap itself, rather than an unrecognized confounding factor, that is associated with risk of diabetes mellitus.