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Review
. 2023 Sep 26;13(10):1968.
doi: 10.3390/life13101968.

The Influence of Light Wavelength on Human HPA Axis Rhythms: A Systematic Review

Isabella Robertson-Dixon  1 , Melanie J Murphy  1 , Sheila G Crewther  1   2 , Nina Riddell  1
Affiliations
Review

The Influence of Light Wavelength on Human HPA Axis Rhythms: A Systematic Review

Isabella Robertson-Dixon et al. Life (Basel). .

Abstract

Environmental light entrains many physiological and behavioural processes to the 24 h solar cycle. Such light-driven circadian rhythms are centrally controlled by the suprachiasmatic nucleus (SCN), which receives information from the short-wavelength-sensitive intrinsically photosensitive retinal ganglion cells. The SCN synchronizes local clocks throughout the body affecting sleep/wake routines and the secretion of neuroendocrine-linked hormones such as melatonin from the pineal gland and cortisol via the hypothalamic pituitary adrenal (HPA) axis. Although the effects of light parameters on melatonin have been recently reviewed, whether the experimental variation of the spectral power distribution and intensity of light can induce changes in cortisol rhythms remains unclear. Thus, this systematic review evaluated the effects of daytime exposure to lights of different spectral wavelength characteristics and luminance intensity on the cortisol levels in healthy individuals. A search of the PubMed, Web of Science, EMBASE, CINAHL, Medline, PsycINFO and Cochrane Library databases on 19 June 2023 identified 3418 articles, of which 12 studies (profiling 337 participants) met the inclusion and risk of bias criteria. An analysis of the literature indicated that exposure to bright lights of any colour during the late night or early morning can induce significant increases in cortisol secretion relative to time-matched dim light comparison conditions. Furthermore, exposure to bright lights with stronger short-wavelength (blue/green) components in the early morning typically induced greater increases in cortisol relative to lights with stronger long-wavelength (red) components. Thus, the circadian regulation of cortisol is sensitive to the wavelength composition of environmental lighting, in line with the more commonly studied melatonin. As such, wavelength characteristics should be optimized and reported in light intervention studies (particularly for the investigation of cortisol-associated disorders and HPA axis function), and exposure to short-wavelength light during sensitive periods should be carefully considered in constructed environments (e.g., bedroom and classroom lighting and device screens).

Keywords: HPA axis; circadian rhythm; cortisol; hypothalamic pituitary adrenal axis; spectrum; stress; wavelength.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 2
Figure 2
Normal relationship between plasma melatonin and cortisol levels and sleep (grey shading) in humans across the 24 h day. Adapted from Hickie et al. [27].
Figure 1
Figure 1
The circadian system: central and peripheral system communication through neural and hormonal interactions. Luminance information from the ipRGCs of the retina in the eye is passed to the suprachiasmatic nucleus (SCN) through the retino-hypothalamic tract (RHT), which differentially influences the synchronization of circadian rhythms throughout the body at different times of day. Neurons transmit this timing information to other parts of the brain and the intrinsic circadian clocks of peripheral organs, thus influencing activities such as hormone secretion, food intake, sleep and regulation of body temperature.
Figure 3
Figure 3
PRISMA flow diagram of the study selection process.
Figure 4
Figure 4
Radial tree displaying the percentage change in cortisol during or following experimental light interventions relative to a pre-light baseline. Inner nodes indicate the time of experimental cortisol measurement. Middle nodes indicate the type of light exposure (dim, white, red, blue or green light). The outer ring of nodes provides a bar graph indicating the percentage change in cortisol during or following the light exposure relative to a pre-light baseline. The percentage change is also displayed numerically in bold next to the study author’s name. Statistical significance as reported by the study authors is indicated by the bar shading and outline. Refer to Table 7 for author citations.
Figure 5
Figure 5
Radial tree displaying the percentage difference in cortisol measurements between time-matched experimental lighting conditions. The inner (first) nodes indicate the time of cortisol measurement (with minutes rounded to the nearest hour). Where multiple cortisol measurements were taken within a short timeframe, the data are summarized in 2 h. blocks. The second and third rings of nodes indicate the two experimental lighting conditions being compared (dim, white, red, blue or green light). In addition to the dim light controls, where coloured lights were substantially brighter or dimmer than their comparator counterparts, this is indicated with an additional black annulus around the coloured dot. The outer (fourth) ring of nodes provides a bar graph representing the percentage change in cortisol between the two experimental light exposures. The percentage change is also displayed numerically in bold next to the study author’s name. Statistical significance as reported by the study authors is indicated by the bar shading and outline. One study [45] only presented descriptive statistics for the average of cortisol measurements collected at 09:00, 13:00 and 17:00 (daytime) and 21:00, 01:00 and 05:00 (night-time). In this study, the night-time measures are expected to be affected by participant sleep deprivation. Two studies [40,74] reported insufficient descriptive statistics to enable calculation of percentage change in cortisol levels between conditions. Thus, their results were unable to be represented in this figure and are described in the manuscript text only. Refer to Table 7 for author citations.
Figure 6
Figure 6
Radial tree displaying the percentage difference in melatonin measurements between time-matched experimental lighting conditions. The inner (first) nodes indicate the time of melatonin measurement (with minutes rounded to the nearest hour). Where multiple melatonin measurements were collected within a short timeframe, the data are summarized in 2 h. blocks. The second and third rings of nodes indicate the two experimental lighting conditions being compared (dim, white, red or blue light). In addition to the dim light controls indicated, where coloured lights were substantially brighter or dimmer than their comparator counterparts, this is indicated with an additional black annulus around the coloured dot. The outer (fourth) ring of nodes provides a bar graph indicating the percentage difference in melatonin between the two experimental light interventions (with the exact value indicated in bold above the bar) and the study author. Statistical significance as reported by the study authors is indicated by the bar shading and outline. One study [45] only presented descriptive statistics for the average of melatonin measurements collected at 09:00, 13:00 and 17:00 (daytime) and 21:00, 01:00 and 05:00 (night-time). In this study, the night-time measures were expected to be affected by participant sleep deprivation. One study [40] reported insufficient descriptive statistics to enable calculation of percentage change in melatonin levels between conditions. Thus, this study’s results were unable to be represented in the figure and are described in the manuscript text only. Refer to Table 7 for author citations.
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