## 3 Discussion
In real-life conditions, 24 h rhythms in TC and HDL-C have a phase in the early evening (peaking around 16:00-17:00 h). A phase advance of approximately 2 h (to around 14:00-15:00) was reported after the age of 60 in healthy adults in India [47], with similar results in Spain [48], later confirmed with a higher-density sampling in the US [49]. A study in Japan found that both LDL-C and HDL-C decrease during the day, both fractions being significantly lower around midnight than after breakfast [50]. Another study conducted in the Republic of North Macedonia reported no 24 h rhythm in HDL-C [51]. The partly contradictory results may be explained by high intra-individual variability in 24 h and 12 h phasing [52] and high inter-individual [49] variability, and by differences in phenotypes of circadian patterns of lipid metabolism. Such differences may depend on the circadian phase of the intrinsic clock [6; 15], which governs sleep and food consumption, and therefore must either directly or indirectly affect circadian profiles of lipid metabolism. Of note, postprandial responses of serum lipids to the same meal depend on time, with the effect in the morning higher than in the afternoon [14]. Furthermore, altered lipid metabolism, higher cholesterol, LDL-C [52], and atherogenic ratio (LDL-C/HDL-C) were found in evening chronotypes [53; 54]. Recently, a constant routine study was conducted [15] that supported early evening mean phases for circadian rhythms of TC (18:32), LDL-C (17:00), and HDL-C (19:17). This study reported that the nocturnal phase for TG (03:22) was very close to that of melatonin (03:41) [15]. The authors also showed phase response curves (PRCs) for lipids (TC, HDL-C, and TG) in response to light and food, with greater shifts for lipids compared to melatonin, approximately in antiphase to melatonin [15]. These findings provide strong evidence that ambient light drives circadian patterns and phases of serum lipids. Interestingly, this study also showed that, unlike other lipids, LDL-C failed to demonstrate a PRC in response to light, suggesting that the mechanisms driving LDL-C are somewhat distinct. This result is intriguing since in our study, morning LDL-C also failed to demonstrate a robust correlation with RGC loss, whereas other lipids did. Moreover, only LDL-C showed MEGs that were attributed to a certain _CLOCK_ gene SNP. Another source of evidence that blood lipids depend on ambient light conditions stems from studies demonstrating differences in serum lipids in photoperiod conditions of different seasons and latitudes. For example, LDL-C, HDL-C, and its turnover by lipase enzymes have seasonal differences, depending on photoperiod or light exposure [30; 55]; reviewed in [31]. There are many discrepancies, especially in TC and LDL-C concerning the season when their peak phase occurs. Gender and age also matter. However, most studies agree that the lowest values are usually seen during the summer, the season also corresponding to the highest light exposure. One study found that summer, the season with an increased light phase, is linked to lower LDL-C and higher HDL-C [55], favoring the hypothesis that the amount of light received per 24 h via the retina could be associated with bidirectional changes in cholesterol fractions. As the lowest value for LDL-C occurred in summer while that for HDL-CL occurred in winter, such reciprocal seasonal differences between LDL-C and HDL-C performed in a location with relatively narrow seasonal differences in ambient temperature (Spain) suggest that, yet again, such differences can be linked to a near 6 h difference in sunset between summer and winter. The association of MEG in LDL-C with the _CLOCK_ gene reported herein further strengthens the evidence that lipid metabolism is clock- and light-controlled. Numerous previous studies showed that the _CLOCK_ gene is involved in lipid metabolism [56], and that its CLOCK_3111 TC (rs1801260) polymorphism matters particularly [38; 57; 58; 59; 60; 61; 62; 63].