From the linear analysis without controlling for any variables, we observed a negative significant association between insomnia alone (β = −1549.38, p-value = 0.004) and the co-existence of insomnia and cognitive decline (β = −1511.71, p = 0.011) with BDNF concentration, suggesting that people with insomnia alone or the combination of both (insomnia and cognitive decline) exhibit a decreased concentration of BDNF (Table 2). Here, BDNF is linked to insomnia.