Numerous studies across multiple animal models suggest that apigenin has connections with sleep and/or with its underlying neurobiology (Table 1). The invertebrate model Drosophila melanogaster can be used to study aspects of Parkinson’s disease (PD), a neurodegenerative disease characterized by brain aggregates of alpha-synuclein and disrupted sleep (53). In a 2017 study, an alpha-synuclein-expressing transgenic D. melanogaster model of PD was used to assess the effects of dietary supplementation with apigenin on PD symptoms and biomarkers. Apigenin- supplemented animals demonstrated a reversal of many biochemical changes characteristic of PD, including alterations in glutathione-S- transferase activity and lipid peroxidation as well as levels of glutathione and oxidative stress (54). Findings germane to oxidative stress are intriguing given that, in flies, sleep deprivation causes early death via the accumulation of reactive oxygen species (55). Apigenin was also reported to decrease activity of monoamine oxidase and increase levels of dopamine (54). It would be interesting to know if this finding is reproducible in normal flies and in more complex rodent models. In rodents, supplementation with apigenin increased sedation or reduced locomotor activity in rodents (56, 57), factors that influence sleep latency. Similar experiments in mice also suggested that apigenin can protect against neurodegeneration while improving learning, memory, neurovascular activity, and levels of several key biomarkers, including Bdnf, Trkb, and Creb, all of whose levels are influenced by sleep quality (58).

Clinical studies assessing apigenin’s potential effects on sleep are promising, albeit limited (Table 2). Many of these studies utilize chamomile extract, which is often about 1% apigenin by mass (0.8–1.2%) and in which apigenin is a bioactive ingredient. It is difficult to be certain whether the effects observed from chamomile extract in these studies are solely due to apigenin, other components of chamomile, or a combination of both. However, these results, combined with the animal data described above, suggest an important role for apigenin.

One study testing the effects of 270 mg of chamomile extract versus placebo twice per day for 28 days in patients with primary insomnia observed a trend toward improvement in daytime functioning, though it did not reach statistical significance (59). Another trial assessed the impact of chamomile extract versus placebo for eight weeks in patients with depression with or without anxiety. Subjects took a single 220 mg capsule daily in the first week and gradually increased this to five capsules (1100 mg) per day for the last four weeks. The authors observed significant reductions in depression and mood scores (60). While not measuring sleep directly, these mental health components are known contributors to sleep latency and quality, and vice versa (61). Additional studies have been conducted on multiple timescales assessing the potential of chamomile to treat anxiety. In one study, 1500 mg chamomile extract was given daily for eight weeks to two groups of patients, one with generalized anxiety disorder (GAD) and the other with GAD and comorbid depression. Chamomile extract reduced anxiety levels in both groups and attenuated depressive symptoms in the comorbid depression group (62). In a longer-term study, 500 mg chamomile extract was given three times daily for 12 weeks to patients with a primary diagnosis of moderate-to-severe GAD. Those whose GAD symptoms were ameliorated in response to treatment were then split into continued treatment versus placebo groups for 26 weeks. In addition to showing improved GAD symptoms during follow-up, chamomile-treated subjects also displayed significant reductions in body weight and mean arterial blood pressure (63).

Drinking chamomile tea after birth for two weeks also caused significant improvements in sleep efficiency and postnatal depression (64). These benefits largely disappeared four weeks post-test, though this could be due to broader changes in physiological activity weeks after birth, rather than a loss in efficacy of chamomile itself (65). Further, a topical formulation of chamomile containing chamazulene (an aromatic compound found in chamomile and related species) and apigenin was tested as a pain relief treatment in patients with migraine (66, 67), which can be a driver of poor sleep (67). Topical chamomile was found to significantly reduce pain, vomiting, nausea, phonophobia, and photophobia in patients with migraines (66). The analgesic properties may in part, be due to its reported ability to inhibit iNOS, PGE2, and COX2 (68). Further evidence comes from a recent cross-sectional study, which sought to identify associations between dietary levels of individual polyphenols and sleep quality. The flavonoid polyphenols apigenin and naringenin were both found to be significantly correlated with sleep quality. Specifically, a low level of dietary apigenin intake was associated with worse sleep quality (69). It would be interesting to better understand which of these effects are due to apigenin on its own or apigenin in conjunction with other molecules in chamomile.

Mechanistic studies suggest several mechanisms by which apigenin could increase sleep quality and quantity (Table 1). In Drosophila, dietary supplementation with apigenin decreased oxidative stress and acetylcholinesterase activity (70), reduced glutathione-S- transferase activity and lipid peroxidation, and increased glutathione levels, all of which are thought to support healthy sleep (71, 72). Apigenin supplementation in rodents decreased corticosterone levels and increased levels of Bdnf (73), CREB, phosphorylated CREB, and serotonin in the hippocampus (74, 75). These findings support the hypothesis of a pro-sleep role for apigenin, as the stress response is known to impair sleep (76). Moreover, BDNF, CREB, phosphorylated CREB (phospho-CREB), and serotonin have all been shown to be sleep-promoting, and BDNF, phospho-CREB, ACh, and TrkB were all found to be increased after apigenin treatment in a mouse model of amyloid beta (Aβ)-driven neurodegenerative disease (58).

In rats, apigenin demonstrated GABAergic activity that was independent of GABA- benzodiazepine receptors and possibly mediated by the GABA_A receptor (57). Apigenin also decreased levels of Tnf-α, Il-6, and iNos1 while maintaining elevated levels of Bdnf and glial Gdnf mRNA (77, 78). Findings involving BDNF are interesting given that this protein was reported to be significantly lower in insomniac patients with short sleep duration (79). Reports of apigenin ameliorating inflammatory markers are similarly intriguing, given a recent study showing that protracted sleep deprivation leads to severe inflammation in the mouse brain (80). While these studies suggest a prominent link between apigenin and sleep biomarkers, very little research has been done explaining how apigenin causally regulates sleep-relevant pathways. As such, future research is warranted to better understand the mechanistic relationship between apigenin and sleep health.

Given the evidence described above, it is important to note the close connection and interdependence between aging and sleep. Poor sleep can accelerate aging, as indicated by its alteration of age-associated epigenetic biomarkers (81) and its strong correlation with mortality risk (82) and healthspan (83). In turn, aging often leads to reduced sleep quality (84), producing a vicious cycle between the two. However, if poor sleep accelerates aging, which further worsens sleep, then improved sleep could improve aging, which would mitigate sleep’s decline. Thus, the interdependence between sleep and aging may complicate our understanding of apigenin’s effects. Longevity benefits reported in animal models could, for example, be due in part to improvements in sleep, a well-established reparative process. While this may be true, a closer look at the mechanistic data reported suggests that apigenin can directly mitigate established hallmarks of aging. Apigenin’s ability to act on a diversity of targets and processes makes it more likely that aging and sleep are largely being independently influenced.

At least some component of apigenin’s efficacy may be due to apigenin-derived metabolites generated by the intestinal microbiome, wherein different products may each contribute to specific elements of apigenin’s effects once entered into circulation. The microbiota has well-described roles in healthy sleep and aging, and dysbiosis is a known contributor to declines in both (85, 86). Indeed, dysbiosis is one of the 12 formally recognized hallmarks of aging (87). Changes in microbial composition could also at least partially explain the interdependence of sleep and aging, alongside other key factors like epigenetics and signaling molecules. Poor sleep quality and aging both alter distributions of microbial species, which alter the distributions of the end products of microbial chemistry and their resulting effects (88, 89). In addition, dietary supplementation with probiotics and targeted lifestyle modifications aimed at improving intestinal flora have been reported to improve sleep quality and mitigate immune and inflammatory aspects of the aging phenotype (90). While it’s clear that sleep can influence aging and vice versa, additional research is needed to understand this relationship on a deeper level. Moreover, it would be intriguing to learn if apigenin’s ability to influence aging (described in detail below) is due, at least in part, to effects on sleep.

Numerous research studies have examined the ability of apigenin to impact lifespan and/or age-related disease (Table 1). One model in Drosophila employs D-galactose, a compound that accelerates aging, shortens lifespan, and increases both oxidative stress and lipid peroxidation (91). In this model, apigenin partially reversed the lifespan-shortening effects of D-galactose (92). In a separate transgenic Drosophila model of Alzheimer’s disease (AD) expressing Amyloid Beta-42 in neurons and producing AD-like amyloid beta aggregates, a daily diet containing apigenin for 30 days was shown to delay the loss of climbing typically seen in this model (70). Apigenin similarly increases survival in a fly model of PD (54) and, in Caenorhabditis elegans, dietary supplementation with apigenin or apigenin glycosides increases lifespan (93, 94).

Promising results have also been documented in vertebrate animal models. In mice, intraperitoneal injection of apigenin before lipopolysaccharide injection strongly blunts the pro-inflammatory and immunomodulatory effects of lipopolysaccharide that are otherwise lethal at high doses (95). In diabetic mice, subcutaneous injection of apigenin improved cardiometabolic health and thyroid function (96). Intraperitoneal injection of apigenin improved glucose and lipid homeostasis in obese mice (97). Further, a diet supplemented with apigenin restored cardiac function and reduced the risk for isoproterenol-induced myocardial infarction in rats with streptozotocin-induced diabetes (98). In a separate mouse model of cardiomyopathy, apigenin delivered via gavage improved heart function and suppressed cardiac inflammation (99). Similarly, in a mouse model of atherosclerosis and non-alcoholic fatty liver disease, dietary apigenin reduced atherosclerosis, hepatic lipid accumulation, body weight, and plasma lipid levels (100). In addition, giving older mice apigenin in drinking water was shown to downregulate genes associated with immune activation and improve both learning and memory (101). In a recent study using a mouse xenograft model of triple-negative breast cancer, daily intraperitoneal injection of apigenin increased apoptosis in tumor cells and reduced tumor proliferation (102).

Clinical studies have yet to directly investigate apigenin’s ability to influence aging. Some studies have assessed apigenin more directly, though mostly in connection to sleep, depression, pain, and anxiety, as described previously (Table 2). While sleep and mental health quality are significant contributors to aging and health (84, 103), human studies assessing apigenin’s potential effect on established biomarkers of aging, such as markers of cardiometabolic health, motor function, and cognition, are certainly needed. Some clinical studies have utilized other flavonoids chemically similar to apigenin, such as quercetin. With the exception of quercetin’s two additional hydroxyl groups, quercetin and apigenin are chemically identical (104). Other research has employed wider varieties of non-flavonoid polyphenols, including phenolic acids, lignans, and stilbenes (105), and many molecules across the flavonoid and non-flavonoid polyphenol classes have been reported to target hallmarks of aging, including cellular senescence (104).

As described heretofore, one of apigenin’s better-characterized functions is to increase levels of NAD⁺ via inhibition of the NADase enzyme CD38. Specifically, mouse NAD⁺ levels in the liver were nearly doubled in response to apigenin (97). The role of NAD⁺ in health and longevity is beyond the scope of this review but has been comprehensively described elsewhere (106). Previous work has shown that NAD⁺ levels decline with age (107), including in specific tissues such as the brain (108). This is due in part to increased consumption and decreased production and salvage of NAD⁺ (109, 110). Older humans with higher levels of NAD⁺ relative to their age group tend to be healthier and exhibit fewer age-associated disease phenotypes (111). While NAD⁺ levels often decline with age in human skeletal muscle, exercise-trained older individuals display NAD⁺ levels comparable to younger subjects (112).

Clinical studies indicate that NAD⁺ levels can be modulated by behavior change and/or supplementation. In humans and mice, exercise of moderate intensity and resistance training appears to increase levels of circulating NAD⁺, NADH, and NAMPT (113, 114), providing one of several possible biochemical explanations linking exercise to healthy aging. Direct supplementation in humans with NAD⁺ precursors may also offer benefits. Analogs of nicotinic acid, an NAD⁺ precursor also known as niacin or vitamin B3, improved mitochondrial function in skeletal muscle in patients with diabetes (115). Nicotinamide mononucleotide, a precursor to NAD⁺, was reported to be safe and well-tolerated (116), increase NAD⁺ levels (117), augment insulin sensitivity in muscle (118), enhance energy (119), and reduce blood pressure (120). Nicotinamide mononucleotide’s effects were found to be more mixed on grip strength (117, 121) and walking ability (121, 122). Nicotinamide riboside, another NAD⁺ precursor reported to be safe and well-tolerated (123), was similarly shown to increase levels of NAD⁺ (124). Nicotinamide riboside was also reported to decrease inflammation and enacted transcriptional changes pertinent to metabolism and mitochondrial function in healthy and overweight individuals (125). While exciting, larger, longer-term clinical trials are needed to better understand the safety and efficacy of direct NAD⁺ precursors like nicotinamide mononucleotide and nicotinamide riboside.

While apigenin may be positively influencing aging by elevating NAD⁺ levels, it could also have other systemic pro-longevity effects (Table 1). Apigenin mitigated D-galactose-driven accelerated aging in Drosophila by attenuating oxidative stress, reducing the opening of the mitochondrial permeability transition pore, and reducing the activity of caspases 9 and 3 (92). Concomitant with slowing disease progression in a transgenic fly model of AD, apigenin decreases oxidative stress, lowers acetylcholinesterase activity, and inhibits the formation of amyloid beta-42 aggregates (70). Long-lived worms treated with apigenin glycosides exhibited reduced reactive oxygen species levels and altered expression of numerous genes, including daf-2, daf-16, sod-3, hsp-16.2, skn-1, gst-4, gcs-1, jnk-1, and sir-2.1. Apigenin glycosides also promoted the translocation of daf-16 and skn-1 into the nucleus (93). In a more recent worm study, apigenin was shown to prolong lifespan by transiently inhibiting mitochondrial respiration and temporarily increasing reactive oxygen species levels. This, in turn, triggered a mitohormetic response which led to an overall increased capacity to deal with oxidative stress. Life extension also depended on the genes aak-2, daf-16, and skn-1 (94). Such findings are interesting given that, in a mouse model of myocardial injury, the protective effects of apigenin were dependent on the mitochondrial unfolded protein response (99).

In lipopolysaccharide-stimulated mouse macrophages, apigenin inhibits the production of the proinflammatory cytokines Il-1β, Il-8, and Tnf. This flavonoid also engages in non-canonical regulation of NF-κB transcription factor activity through hypophosphorylation of Ser536 in the p65 subunit and the inactivation of the lipopolysaccharide-stimulated IKK complex (95). Apigenin has also been reported to improve glucose and lipid homeostasis in mice by increasing levels of insulin and thyroid hormone and reducing levels of glucose, glucose-metabolizing liver enzymes, and cholesterol (96). Further, Cd38 knockout mice fed a high-fat diet have higher levels of NAD⁺, are less likely to develop obesity and metabolic syndrome, and have increased survival compared to wild-type animals (97). In contrast, Parp1 knockout mice show worse survival on a high-fat diet. This may be due to the role Parp1 plays in DNA repair and genomic stability (97). Additionally, in Ldlr and Nlrp3 knockout mice fed a high-fat diet, apigenin appeared to reverse the cardiac and hepatic symptoms of the Ldlr^–/– genotype in an inflammasome-dependent manner, as the apparent benefits of apigenin were abrogated in the double knockout, and treatment of liver cells cultured in vitro demonstrated consistent findings (100).

Apigenin’s cognitive effects may be mediated by specific cell types. Older mice given apigenin in drinking water experienced glial cell-associated transcriptional changes across immunity, inflammation, and cytokine regulation into expression profiles that were characteristic of younger animals, and they also exhibited reduced inflammation and cellular senescence in astrocytes (101). In a recent study using immuno-deficient mice implanted with human tumor cell xenografts, apigenin appeared to induce transcriptome-wide reprogramming of cancer-associated alternative splicing in an RNA binding protein (hnRNPA2)-associated manner and induce switching of cancer-associated to non-cancer-associated alternative spliced isoforms (102). Studies in rats also allude to potential mechanisms of action for apigenin. Work by Ogura et al showed that, in a rat model of diabetic kidney disease, apigenin ameliorated renal injuries, pro-inflammatory gene expression, and tubular cell damage. In the kidney, apigenin also elevated the NAD⁺/NADH ratio and enhanced the activity of Sirt3 (126). In rats, protein expression of the nuclear receptor Pparg in the myocardium was elevated by apigenin (98).