Circadian regulation of glucose, lipid, and energy
metabolism in humans
Eleonora Poggiogalle
a
, Humaira Jamshed
b
, Courtney M. Peterson
b,
⁎
a
Department of Experimental Medicine, Medical Pathophysiology, Food Science and Endocrinology Section, Sapienza University, Rome, Italy
b
Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA
ARTICLE INFO ABSTRACT
Article history:
Received 7 July 2017
1 November 2017
Accepted 24 November 2017
The circadian system orchestrates metabolism in daily 24-hour cycles. Such rhythms
organize metabolism by temporally separating opposing metabolic processes and by
anticipating recurring feeding-fasting cycles to increase metabolic efficiency. Although
animal studies demonstrate that the circadian system plays a pervasive role in regulating
metabolism, it is unclear how, and to what degree, circadian research in rodents translates
into humans. Here, we review evidence that the circadian system regulates glucose, lipid,
and energy metabolism in humans. Using a range of experimental protocols, studies in
humans report circadian rhythms in glucose, insulin, glucose tolerance, lipid levels, energy
expenditure, and appetite. Several of these rhythms peak in the biological morning or
around noon, implicating earlier in the daytime is optimal for food intake. Importantly,
disruptions in these rhythms impair metabolism and influence the pathogenesis of
metabolic diseases. We therefore also review evidence that circadian misalignment
induced by mistimed light exposure, sleep, or food intake adversely affects metabolic
health in humans. These interconnections among the circadian system, metabolism, and
behavior underscore the importance of chronobiology for preventing and treating type 2
diabetes, obesity, and hyperlipidemia.
© 2017 Elsevier Inc. All rights reserved.
Keywords:
Circadian
Diurnal rhythm
Circadian misalignment
Meal timing
1. Introduction
The circadian system organizes metabolism, physiology, and
behavior in a daily cycle of circadian rhythms. Circadian derives
from the Latin roots circa meaning around and diēm meaning
day, and like all daily or diurnal rhythms, circadian rhythms
are periodic patterns that repeat themselves approximately
every 24 h. However, unlike diurnal rhythms, circadian
rhythms are generated endogenously within the organism
and perpetuate themselves even in the absence of external
time cues (Fig. 1). Such circadian rhythms have evolved over
hundreds of millions of years to orchestrate metabolism by
temporally separating opposing metabolic processes (such as
anabolism and catabolism) and by anticipating recurring
feeding-fasting cycles to optimize metabolic efficiency [1–3].
The circadian system comprises a central pacemaker in
the brain and a series of clocks in peripheral tissues
throughout the body, including liver, muscle, and adipose
METABOLISM CLINICAL AND EXPERIMENTAL 84 (2018) 11– 27
Abbreviations: CREB, cAMP response element binding protein; AMPK, adenosine monophosphate-activated protein kinase; SCN,
suprachiasmatic nucleus; TTFL, transcriptional-translational feedback loop; CR, constant routine; FD, forced desynchrony; CA/M,
circadian alignment/misalignment; h, hour; AUC, area under the curve; IRS-1, insulin receptor substrate-1; FFAs, free fatty acids; RQ,
respiratory quotient; TEF, thermic effect of food; RCTs, randomized controlled trials; BMI, body mass index.
⁎ Corresponding author at: University of Alabama at Birmingham, 1720 2nd Avenue South, Webb 644, Birmingham, AL 35294, USA.
E-mail address: cpeterso@uab.edu (C.M. Peterson).
YMETA-53688; No of Pages 17
https://doi.org/10.1016/j.metabol.2017.11.017
0026-0495/© 2017 Elsevier Inc. All rights reserved.
Available online at www.sciencedirect.com
Metabolism
www.metabolismjournal.com
tissue. This system of clocks collectively modulates a wide array
of metabolic targets, such as glucocorticoids [4],themaster
energy sensor AMPK [5], rate-limiting steps in fatty acid and
cholesterol synthesis [6,7], and hepatic CREB to modulate
gluconeogenesis [8]. The aggregate effect is that an array of
metabolic processes—including insulin sensitivity, insulin secre-
tion, cholesterol synthesis, fat oxidation, and energy
expenditure—all follow a rhythm across the 24-hour day [2,3,9].
In addition to evidence of circadian rhythms in metabolism,
data increasingly suggest that disruption of the circadian system
increases the risk of metabolic diseases [9–12].Inrodentstudies,
clock gene mutants often display obese or diabetic phenotypes
and possess defects in core metabolic pathways such as insulin
secretion and gluconeogenesis [3,13–17].Moreover,misalignment
of circadian rhythms in rodents often makes them hyperphagic,
insulin resistant, and hyperlipidemic [9–12].Inhumantrials,
circadian misalignment similarly elevates glucose, insulin, and
triglyceride levels [18–20] and lowers energy expenditure [21].
Therefore, understanding these rhythms is important for timing
when to eat, sleep, be exposed to bright light, be physically active,
and even when to take medications to reduce the risk of
metabolic diseases [22–24].
While there is ample mechanistic data in animal models
demonstrating the wide-sweeping role of the circadian system in
metabolism, there are comparatively fewer trials in humans.
Given that rodents differ in several key ways from
humans—such as being nocturnal, polyphasic (sleeping mo re
than once per day), and having high metabolic rates per body
weight—it is unclear how, and to what degree, circadian and
diurnal research in rodents translates into humans. In this
review, we sy nthesize evidence for circadian regulation of
metabolism in humans. In Section 2, we provide an overview of
the architecture of the circadian system and protocols for
measuring circadian rhythms in humans. In Section 3,we
summarize the evidence for circadian and diurnal rhythms in
glucose, lipid, and energy metabolism in humans. In Section 4,
we conclude by discussing how circadian alignment or misalign-
ment with three external factors—light,sleep,andfood
intake —affects metabolism and the risk of metabolic diseases.
2. Circadian Biology
2.1. Architecture of the Circadian System
The circadian system consists of two parts: (1) a central clock
located in the suprachiasmatic nucleus (SCN) of the hypo-
thalamus and (2) a series of peripheral clocks located in
Diurnal Rhythms Glossary.
Diurnal Rhythm: Any physiologic, behavioral, or other biological rhythm that repeats itself
approximately every 24 hours.
Circadian Rhythm: Any diurnal rhythm that is endogenously generated by an organism and
that sustains itself even in the absence of light and other external cues.
Mesor: The midline or mean of a rhythm.
Peak: The highest value of a rhythm.
Trough or Nadir: The lowest value of a rhythm.
Amplitude: The magnitude or strength of a rhythm. This can be expressed as either a peak-to-
mesor amplitude (as shown below) or a peak-to-trough amplitude. (In this review, we report
amplitudes predominantly as peak-to-trough amplitudes.)
Phase: The timing of a rhythm, which is defined relative to a key point in the rhythm (typically
the peak or trough).
Acrophase: The time at which a rhythm peaks.
Period: The length of time that it takes a rhythm to repeat itself. Circadian rhythms have
approximately 24-hour periods.
Entrainment: The synchronization of a rhythm to an external or environmental cue.
Zietgeber: An external cue that entrains or influences the phase of a rhythm.
Phase Advance: A shift in the timing of a rhythm such that it begins earlier.
Phase Delay: A shift in the timing of a rhythm such that it begins later.
Alignment: The difference in phases between any two rhythms.
Misalignment or desynchrony: The state of having an abnormal alignment or difference in
phases between two rhythms. The two rhythms are said to be misaligned or mistimed.
Transcriptional-Translational Feedback Loop (TTFL): A series of feedback loops among
circadian clock genes and proteins that maintain the ~24 hour rhythms in nearly every cell of
the body.
Phase
(timing)
Amplitude
Time of Day
Peak
Trough
Structure of a Diurnal Rhythm
Fig. 1 – Diurnal rhythms glossary.
12 METABOLISM CLINICAL AND EXPERIMENTAL 84 (2018) 11– 27
virtually all other tissues of the body, including the liver,
pancreas, gastrointestinal tract, skeletal muscle, and adipose
tissue (Fig. 2). The central clock is thought to regulate
metabolism through diffusible factors (primarily cortisol and
melatonin) and synaptic projections (including via the auto-
nomic nervous system) [25,26]. Peripheral tissues integrate
these signals from the central clock with environmental and
behavioral factors (including light, sleep, physical activity,
and feeding) and their own autonomous rhythms to regulate
metabolism in a rhythmic manner [27]. The autonomous
intracellular rhythms are maintained on a molecular level by
clock genes and proteins that form a transcriptional-
translational feedback loop (TTFL). The TTFL operates in a
~24-hour cycle, activating a rhythmic cascade of transcrip-
tional and posttranscriptional events involving thousands of
target genes [28]. In total, about 10% of genetic transcripts
exhibit circadian periodicity, and moreover, an even larger
number of proteins undergo oscillations arising from circadi-
an rhythms at the post-transcriptional and post-translational
levels [28].
The timing (phase) of each circadian rhythm is determined
by external factors in a process known as entrainment. The
central clock's rhythm is primarily entrained by light,
whereas the rhythms in peripheral tissues arise from
integrating inputs from the central clock, external factors
(including light, physical activity, feeding, and sleep), and
metabolites [25,27]. Recently, the timing of food intake has
emerged as one of the key zeitgebers or external factors that
sets the phases of peripheral clocks [29,30]. Because different
stimuli set the phases of the central and peripheral clocks, the
two clock systems become misaligned whenever their respec-
tive zeitgebers are out of sync. This misalignment disrupts
metabolism since the two clock systems jointly coordinate
interdependent metabolic pathways. As we will discuss in
Section 4, several controlled trials now suggest that circadian
misalignment elevates the risk of developing metabolic
diseases.
2.2. Measuring Circadian Rhythms
Measuring the circadian component of a rhythm is challeng-
ing and requires matching or eliminating all time-dependent
external factors in order to isolate the circadian (endogenous)
rhythm [31]. To date, four protocols have been developed for
measuring circadian rhythms in humans (see Fig. 3 for
detailed descriptions) [31,32]. Each of these protocols has
unique advantages and disadvantages. The Constant Routine
(CR) Protocol involves an extended period of wakefulness (no
sleep) longer than 24 h, during which all external factors
(including light, temperature, and feeding) are kept constant.
Fig. 2 – The architecture of the circadian system. The circadian system comprises a central clock, which is located in the SCN of
the hypothalamus, and a series of peripheral clocks located in tissues throughout the body. The central clock is entrained
primarily by light, and its rhythm is measured through frequent sampling of melatonin, cortisol, or core body temperature.
The central clock affects the phases and amplitudes of peripheral clocks through hormones and synaptic projections. The
peripheral clocks are entrained by a combination of these signals from the central clock and external factors, most notably the
timing of food intake. Peripheral clock rhythms are measured in humans either by directly measuring the rhythm in a
physiologic variable or by measuring the expression of clock genes. Overall, daily rhythms in metabolism are produced by the
central and peripheral clocks working in concert.
13METABOLISM CLINICAL AND EXPERIMENTAL 84 (2018) 11– 27
Because all external factors are held constant, any rhythms
observed during a CR protocol are assumed to be pure
circadian rhythms, generated only by the endogenous circa-
dian system. The other three protocols permit external factors
to be present and allow participants to sleep but involve
changing the timing of sleep to cycle through different parts
of the 24-hour day. Mathematical techniques are then used to
extract the circadian component of the rhythm. As a result,
A.
B.
C.
14 METABOLISM CLINICAL AND EXPERIMENTAL 84 (2018) 11– 27
these protocols have the advantage of providing information
on both the circadian and external (behavioral) components
of the rhythm, as well as on the effects of circadian
misalignment. The Forced Desynchrony (FD) Protocol entails
participants typically living on 20- or 28-hour-long days for 1–
2 weeks and allows reconstruction of the full circadian and
behavioral rhythms. By contrast, the Circadian Alignment/
Misalignment (CA/M) Protocol and Inverted Sleep-Wake Cycle
Protocol involve sleeping both during the daytime and at
nighttime and therefore enable estimates of the relative
contributions of the circadian system versus external factors.
In Section 3, we focus on trials using these protocols. Given
the scarcity of such trials, we also draw on studies measuring
diurnal rhythms to provide historical context and to indicate
areas for future inquiry.
3. Circadian and Diurnal Rhythms in Metabolism
3.1. Glucose Metabolism
3.1.1. Diurnal Studies
The first evidence for circadian regulation of glucose metab-
olism emerged in the late 1960’s and 1970’s when several
studies reported diurnal variations in glucose tolerance
[33–47]. Since then, more than a dozen human studies have
reported the existence of a diurnal rhythm in oral glucose
tolerance, typically peaking in the morning, with impair-
ments in glucose tolerance in the afternoon and evening
[33–49]. Importantly, these time-of-day effects are indepen-
dent of the fasting duration [40,48]. Studies using intravenous
glucose or insulin tolerance tests [46,50–54] and mixed meals
[55–62] have reported similar findings. (However, fasting
glucose is usually lower in the afternoon and evening than
in the morning [51,63].) The size of the diurnal variation in
glucose tolerance is strikingly large: adults with normal
glucose tolerance in the morning are metabolically equivalent
to being prediabetic in the evening [33,36,46]. More recently,
we reported that oral glucose tolerance in prediabetic adults
was 40 mg/dl higher at 19:00 h than at 7:00 h, making
prediabetic adults metabolically equivalent to early-stage
diabetics at dinnertime [63].
These diurnal variations in glucose tolerance can be
partially traced to diurnal rhythms in β-cell responsiveness,
insulin secretion, and insulin clearance. Although data on the
existence of a diurnal rhythm in fasting insulin is mixed
[51,64,65], the insulin secretory response varies across the
day. β-cell responsiveness—as measured by glucose toler-
ance, mixed meal, or intravenous tolbutamide testing—is
higher in the morning than at other times of day
[37,46,50,52,55,57,64,66]. Yet, the insulin secretion rate and
the total insulin secreted in response to a meal appears to
peak later in the day [55–57,62,67]. One trial using 68-hour
euglycemic and hyperglycemic clamps found that the insulin
secretion rate peaked in the mid-afternoon (12:00–18:00 h)
and was lowest at night while participants were sleeping [67].
Similarly, trials employing intravenous glucose tolerance
tests and mixed meal tests using C-peptide deconvolution
analysis report that total insulin secretion (AUC of the insulin
secretion rate) is 16–51% higher in the afternoon or early
evening than in the morning, due to a prolonged secretory
period, even when no diurnal rhythm in the peak value of the
secretion rate is apparent [55–57,62]. Insulin clearance also
exhibits diurnal variation: hepatic insulin extraction is lower
in the morning than the evening [55].
Rhythms in peripheral insulin sensitivity also appear to
contribute to the diurnal variation in glycemic control. In one
trial employing a frequently-sampled intravenous glucose
tolerance test in normal-weight participants, insulin sensi-
tivity was impaired by 34% in the evening relative to the
morning [50]. Impairments in insulin sensitivity later in the
day have also been confirmed using insulin tolerance tests
[46,68,69], mixed meal tolerance tests using the triple tracer
technique [55], constant glucose infusion procedures using
isotope tracers [70], and a 24-hour glucose-controlled insulin
infusion procedure reminiscent of a clamp [71]. The diurnal
rhythm in peripheral insulin sensitivity is likely due to both
core intracellular pathways mediating glucose uptake and
circulating factors. About 15% of transcripts in skeletal muscle
exhibit a rhythmic pattern [72], including genes involved in
glucose and lipid metabolism [61,72,73]. Muscle and liver
glycogen content exhibit ~17% peak-to-trough variations,
peaking in the evening [74].
Subcutaneous, but not visceral,
adipose tissue also displays a large-amplitude circadian
Fig. 3 – Four protocols for investigating circadian rhythms in humans. (A) The Constant Routine Protocol involves a greater than
24-hour period of constant wakefulness wherein all external factors (including light, posture, feeding, and temperature) are
kept constant. This protocol allows reconstruction of the entire circadian rhythm but does not enable investigation of circadian
misalignment. (B) The Inverted Sleep-Wake Cycle involves periods of nocturnal and daytime sleep, separated by a prolonged
period of wakefulness. Feeding and posture are typically kept constant throughout the protocol, but light levels are set to
match changes in sleep/wakefulness. This protocol provides insight into both circadian and behavioral cycles. (C) The
Circadian Alignment/Misalignment Protocol includes two subprotocols: the alignment protocol with daytime behavioral cycles
occurring as they would normally, and a misaligned protocol, with those identical behavioral cycles occurring during the
biological night. Light levels are varied, and participants eat normal meals and snacks. While this protocol does not allow
reconstruction of the underlying circadian rhythm, it does reveal how much a diurnal rhythm is influenced by the circadian
phase (i.e., the time of day), circadian misalignment, and behavioral factors. (D) The Forced Desynchrony Protocol involves
living on 20- or 28-hour days for typically 1–2 weeks to cycle through different alignments between circadian rhythms and
behavioral rhythms. Light levels during wakefulness are kept very low, and participants consume normal meals and snacks.
Mathematical procedures are then used to extract the underlying circadian versus behavioral components of the diurnal
rhythm. This protocol also reveals the impact of circadian misalignment on biologic endpoints.
15METABOLISM CLINICAL AND EXPERIMENTAL 84 (2018) 11– 27
