We used high-precision fast spectrum radiometer to characterize the physical property of four artificial lights. As shown in Fig. 1b, the spectra of light (1900 K) is concentrated from 500 nm to 650 nm, presenting yellow-orange-red light. The artificial lights (3000 K, 4000 K and 6600 K) presented a mixed light with the spectra ranging from 300 nm to 780 nm. It is worth noting that the blue light appeared in the lights with the color temperature of 3000 K, 4000 K and 6600 K, except 1900 K, which implied that the artificial light (1900 K) could reduce the adverse effect of blue light to eyes9. In addition, the color rendering index of light (1900 K) is 80 and the luminous efficiency is 66.73 lm/W, which indicated that it is an appropriate lighting source.

Melatonin, an important amino hormone, can improve the sleep quality10,11,12. To evaluate the biological effects of artificial lights on sleep, we monitored the melatonin level of 38 volunteers in all who were required to read under the irradiation of the four artificial lights (Fig. S1a,b), respectively. As shown in Fig. 2a, compared with the lights (3000 K, 4000 K and 6600 K), the average concentration of secreted melatonin is more than 1.5 times in the volunteers who treated with light (1900 K). Besides, we also found that the melatonin level increased by more than 400% in some individuals under the irradiation of light (1900 K), while the melatonin level of volunteers treated with other three light sources increased mildly (Fig. S2), which suggested that the light (1900 K) presented a better positive effect on sleep. To further assess the melatonin secreted behavior, the concentration of melatonin was monitored through measuring the collected saliva from volunteers for 2.5 h, the saliva was collected every 30 min. As shown in Fig. 2b, the results also suggested that the light (1900 K) could promote the secretion of melatonin, and we also found that the concentration of melatonin presented an obvious change after 2 h irradiation. The results showed that the secretion of melatonin presented a mild tendency before 1.5 h, but a sharp tendency after 2 h. It indicated that a large amount of melatonin was secreted after 1.5 h irradiation due to the biological response of human to yellow light. However, the concentration of melatonin changed mildly in volunteers who treated with light sources (3000 K, 4000 K and 6600 K), which indicated that these light sources supplied limited effect on melatonin secretion (Fig. 2c–e). These results provided a valuable reference for the practical application of light source with low color temperature (1900 K) in future.

Figure 2 (a) The melatonin secretion from volunteers treated with artificial lights (1900 K, 3000 K, 4000 K and 6600 K) for 2 h (n ≥ 30). (b–e) The melatonin level of volunteers treated with artificial lights (1900 K, 3000 K, 4000 K and 6600 K) at the desired time (n ≥ 10). (f) The glutamate level of volunteers treated with artificial lights (1900 K, 3000 K, 4000 K and 6600 K) for 2 h (n ≥ 30) ***P < 0.001. Full size image

With the bursting secretion of melatonin, human’s cognitive ability would be interfered due to the sleepiness. Glutamate secretion is a representative factor for characterizing human’s cognitive ability13,14. Therefore, the glutamate level was also measured through the collected saliva from volunteers treated with different light sources (1900 K, 3000 K, 4000 K and 6600 K) (Fig. 2f). Surprisingly, we found that only light source (1900 K) could also promote the glutamate secretion after 2 h irradiation. We then repeated the experiment twice to confirm this result. This information indicated that artificial light (1900 K) could not directly affect work efficiency. We supposed that the molecular mechanisms underlying the effect of light on melatonin and glutamate were different, resulting in the increased melatonin secretion with the accelerative glutamate secretion synchronously11,12,13,14,15.

Therefore, there are several advantages for the light source (1900 K): (1) The appropriate color rendering index (80%) make it to be an ideal normal light source for reading. (2) People could arrange their sleep time by pre-setting the light source (1900 K) irradiation to define the time of bursting secreted melatonin (about 2 h). (3) Before the sleep, the increased glutamate secretion is benefit to human’s cognitive ability.

On the other hand, the eyes are the direct organ to receive lights, and the images were formed in the brain through various processes. Therefore, to evaluate the effect of light on eyes, the non-invasive tear break-up time (NITBUT) and the average NITBUT were assessed through the same groups of volunteers after treating with different light sources (1900 K, 3000 K, 4000 K and 6600 K), respectively. As shown in Fig. 3a,b, the NITBUT was increased in the group of light source (1900 K), while NITBUT was decreased in the other three groups. It suggested that the stability of tear film was better in the group of light source (1900 K). Then we measured the tear meniscus height and red eyes analysis. The height of tear meniscus increased in the group of light sources (1900 K, 3000 K) (Fig. 3c) and the level of red eye was downgraded only in the group of light sources (1900 K) (Fig. 3d,e). These results also suggested that the light sources (1900 K) could relieve eye drying or red eye. We speculated that the wavelength of light sources (1900 K) was distributed near the red light and lacked the blue light, which made the light sources (1900 K) to be harmless to eyes.

Figure 3 The biological effect on human’s eyes before and after irradiation by artificial lights (1900 K, 3000 K, 4000 K and 6600 K), the experiment was repeated by three times at different time. (a) The first non-invasive tear break-up time. (b) The average non-invasive tear break-up time. (c) The tear meniscus height. (d) The red eye analysis (B grading). (e) The red eye analysis (L grading). Full size image

In the previous studies, red light was reported to be able to promote the wound healing and hair regeneration16,17,18,19. Besides, red light was used to promote tissue regeneration in burn and dermatology department16,20. In this study, the proposed light source (1900 K) has a large area of red light wavelength, so we hypothesized that light source (1900 K) also could promote wound healing and hair regeneration. Therefore, we evaluated the effect of light sources (1900 K, 3000 K, 4000 K and 6600 K) on wound healing (Fig. 4) and hair growth (Fig. 5) through building human and murine wound model and murine alopecia model respectively.

Figure 4 (a) The murine wound models were treated with different artificial lights (1900 K, 3000 K, 4000 K and 6600 K). (b) The human’s skin was irradiated by different artificial lights after being treated with microneedles (Scale bar: 20 μm). (c) The wound healing time of mice (n ≥ 3). (d) The wound healing time of human (n ≥ 4) ***P < 0.001. Full size image

Figure 5 The hair regeneration assessment in murine alopecia model, the area of mice was irradiated by different artificial lights (1900 K, 3000 K, 4000 K and 6600 K) after being treated with stem cells growth factors (n ≥ 3) (Scale bar: 20 μm). Full size image

To build the wound model, microneedles were used to press on skin, then the volunteers or mice were irradiated under the different light sources, the healing time was recorded as the skin healing time (Fig. 4a,b). Significant difference was shown in the healing time of human and murine wound model between light source (1900 K) and other groups. The wound healing time presented less than 10 min in the group of light source (1900 K) only (Fig. 4c,d). The results suggested that light source (1900 K) could be used in our normal life to promote small area wound healing.

Red light was reported to be benefit to promote hair regeneration in the previous study17,18,19,20. Therefore, we speculated that the artificial light (1900 K) also could promote hair regeneration. As shown in Fig. 5, the mice were pre-treated by the equivalent amount of stem cells growth factors, which could stimulate hair regeneration21,22. Then the mice were irradiated by different light sources. After 2 weeks, the group of artificial light (1900 K) presented a faster hair growth speed than other three artificial lights (3000 K, 4000 K and 6600 K). It suggested that artificial light (1900 K) also could promote hair regeneration.