“Post-lunch dip” and napping

“Eating lunch has really made me feel really tired.”

“Me too! Perhaps we ate too much or there’s something wrong with the food?!”

Does this conversation sound familiar? What actually drives the “post-lunch dip” in the early afternoon? First, it would be useful to understand the two-process model of the sleep-wake cycle, which was conceptualised by Borbély in 1982.¹

Two-process model of sleep-wake cycle and post-lunch dip

Borbély’s model has two parts: Process C and Process S. Process C can be regarded as the alert drive, which sets a ~24-hour rhythm of sleeping and waking.1 Process S is the sleep homeostatic drive, which can be considered a predictor of sleep pressure. The longer we are awake during the day, the more sleep pressure will be accumulated. Conversely, sleep pressure will gradually dissipate during sleep. The interaction between two processes is thought to regulate our sleep patterns.

In this two-process model, the post-lunch dip can happen when there is an increasing divergence between process S and process C. When the rise in Process S (the sleep pressure) is not counteracted by process C (the alert drive), it is easier to experience sleepiness. Therefore, the post-lunch dip is likely to occur even without a lunch break.

Two-process model involved in the sleep-wake cycle

Process C, the circadian pacemaker and process S, the sleep homeostatic drive.

A common coping strategy for the need for sleep or sleep debt is napping. As sleep pressure will dissipate during sleep, a “power” nap (<20 mins) is hypothesised to improve alertness and cognitive function, while a longer nap may induce sleep inertia that suppresses alertness afterwards.²

Napping can be influenced by cultures (such as a siesta), work schedules (such as night shift work), and other behavioural factors. While there is laboratory evidence for the short-term benefits of cognitive function after a short nap, there is limited epidemiological evidence for various health consequences of napping regularly.

Napping and long-term health consequences

Results of systematic reviews of prospective studies examining daytime napping were inconclusive. A meta-analysis reported a positive association of daytime nappers versus never nappers with the risk of all-cause mortality, but no significant association of daytime nappers with the risk of mortality from cardiovascular disease (CVD), when compared with the referent groups.³ In another meta-analysis, however, long (>60 mins/day) but not short daytime nappers were associated with increased risk of CVD, when compared with no napping individuals.⁴ There is also some evidence suggesting that daytime napping is a risk marker of cancer.⁵

Conclusion and future plans

In short, the post-lunch dip is related to the interaction of two processes in the sleep regulation, and a short nap can help improve our short-term alertness – which may come in useful if you’ve been up all-night coding or trying to write your thesis! However, with respect to the adverse outcomes of regular or habitual napping, more population-based, prospective studies are needed to reliably examine the relationships with major diseases in old age. One focus of my current study aims to examine the associations of daytime napping with the risk of cancer.

Angel Wong is a DPhil student and Clarendon Scholar at the Cancer Epidemiology Unit. Her thesis aims to examine the associations of sleep duration and other sleep patterns with chronic diseases in the Million Women Study and the UK Biobank. Prior to joining the Unit, she completed an MSc in Global Health Science at Oxford, with her dissertation focusing on the association between cardiovascular risk factors and frailty. Before this, she studied a BSc in Public Health in Hong Kong.

References

1. Lockley SW, Foster RG. Chapter 2 Sleep generation and regulation – a framework. In: Lockley SW, Foster RG, editors. Sleep: A Very Short Introduction. Oxford: OUP; 2012. p.16.
2. Faraut B, Andrillon T, Vecchierini M-F, Leger D. Napping: A public health issue. From epidemiological to laboratory studies. Sleep Med Rev.35:85-100.
3. Zhong G, Wang Y, Tao T, Ying J, Zhao Y. Daytime napping and mortality from all causes, cardiovascular disease, and cancer: a meta-analysis of prospective cohort studies. Sleep Med. 2015;16(7):811-9.
4. Yamada T, Hara K, Shojima N, Yamauchi T, Kadowaki T. Daytime napping and the risk of cardiovascular disease and all-cause mortality: a prospective study and dose-response meta-analysis. Sleep. 2015;38(12):1945-53.
5. Cairns BJ, Travis RC, Wang XS, Reeves GK, Green J, Beral V. A short-term increase in cancer risk associated with daytime napping is likely to reflect pre-clinical disease: prospective cohort study. Br J Cancer. 2012;107(3):527-30.