1. Study subjects and animal welfare
Thirty-two healthy young rhesus macaques (Macaca mulatta) (16 male, 16 female) were purchased from the Kunming Primate Research Center, Chinese Academy of Sciences (Yunnan, China). The monkeys were 2.1-2.5 years old (2.35 ± 0.03; mean ± SD), thus approximating primary school-aged children (6-7.5 years) and mimicking the vulnerable stage of ocular development in children.
Before the experiment, all monkeys were reared in a semi-open indoor-outdoor environment under natural sunlight. Each colony had an outdoor stainless-steel cage (2.67 m length) connected to an indoor room (2.61 m wide, 2.46 m long, and 2.58 m high). The monkeys could enter and leave the outdoor cage and indoor room at will. All rearing and experimental procedures were reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of the Kunming Primate Research Center (Approval Number: IACUC20029) and were in strict compliance with the National Care and Use of Animals Guide approved by the National Animal Research Authority (China) and the National Institutes of Health Guide for the Care and Use of Laboratory Animals (USA).
2. Experimental lights, implementation, and data acquisition
2.1 Selection of experimental lights
Correlated color temperature (CCT) describes the proportions of different wavelength components of a light source. Notably, the same CCT can originate from different spectra. To ensure data comparability among different lighting conditions, we selected a commercial LED light with a typical spectrum (Fig. S1A, LED 4000 K) and generated two other LED lights with different CCTs by varying the ratio of their long and short wavelength components (Fig. S1A, LED 3000 K and LED 5000 K). The resulting CCTs of these three groups mentioned above were 2883 ± 30 K, 3803 ± 14 K, 4740 ± 13 K, respectively. For light comparison, we selected a conventional incandescent lamp (IL) with a CCT of 2700 K (Fig. S1B). All four lights were panel lights of the same size (60 × 60 cm) and were mounted on the ceiling about 1 m above each cage.
2.2 Experimental implementation
Before the start of the experiment, all monkeys underwent ophthalmic examination and collection of ocular baseline data (e.g., ocular axial length, keratometry radius, anterior chamber depth, refractive status). During examination, subjects with cataracts or other ophthalmic diseases were excluded. The remaining monkeys were then divided into four groups of eight animals according to their axial lengths, so that the mean values of axial length were comparable among groups (Table S1, F3, 60 = 0.001, P =1.00). At the beginning of the experiment, the four groups were randomly assigned to four experimental rooms with pre-installed experimental lights. All animals were transferred to their rooms on the same day for single-cage housing (each cage was 0.8 m wide, 0.8 m long, and 0.8 m high). Each room housed eight single cages arranged in two columns along each side of the room. Careful adjustments were made so that the illumination (504 ± 9 lx; measured with SFIM-400, Everfine Corporation, Hangzhou, China) at the center of each cage. As the lamps were mounted right on top of each cage, light directly illuminated each cage from the top.
The light cycle was 12 h light/12 h dark per day, with lights on from 07:00 am-19:00 pm. Each animal was provided with free access to food and water, and they were inspected daily by experienced veterinarians.
We did not intervene with visual development of the monkeys but allowed it to develop naturally under artificial lighting in order to mimic the developmental process of the ocular axial in children under indoor activity conditions. During the experiment, illumination at the center of each cage was measured and adjusted at regular intervals (28-32 days) to maintain an illumination of ~500 lx.
2.3 Data collection
Measurements of ocular axial length, corneal curvature radius, anterior chamber depth, and refractive status were performed on the monkeys at regular intervals (every 28-32 days). All data were collected at the same time of day between 12:00 pm and 16:30 pm.
For data collection, each monkey was first anesthetized with ketamine (intramuscular administration, 20-25 mg/kg, Jiangsu Zhongmu Beikang Pharmaceutical Co., Ltd., China) and acepromazine maleate (intramuscular administration, 0.15-0.20 mg/kg, Shanghai Aladdin Biochemical Technology Co., Ltd., China). Ocular axial length, corneal curvature radius, and anterior depth were then measured using an optical biometer (IOL Master, Carl Zeiss Meditec, Germany). Immediately after these measurements, tropicamide phenylephrine eye drops were administered (Mydrin-p, Santen, Osaka, Japan) in each of the monkey’s eyes 5 min apart for mydriatic and ciliary anesthesia, and refractive examination was performed 20 min later using a computerized autorefractor meter (KR-800, TOPCON Medical Systems Inc., Japan). Due to the similarity between monkey and human eyes, measurement of monkey eyes using optical biometry and computerized autorefractometry has been proven feasible and effective 15-17.
3. Statistical analysis
After ocular measurements were complete, data from the two eyes of each monkey were pooled together after statistical analyses showed no significant differences between them. The ocular axial length, corneal curvature radius, and anterior chamber depth data before and after artificial illumination were analyzed and compared by one-way analysis of variance (ANOVA), followed by Bonferroni correction for post hoc tests. The change trends of ocular parameters over time were analyzed and compared using a generalized linear mixed model (GLMM). All statistical analyses were performed using SPSS Statistics v27.0 (IBM Corp., Armonk, NY, USA).