I bring up the sleep quality issue out of personal experience. Years ago a sleep study showed my "sleep architecture" was off - I had little or no deep sleep during the night cycles. They told me this was a natural development as one ages, but deep-sleep deficiencies can contribute not only to excessive daytime sleepiness but also to depression in those people susceptible to it.

Getting good quality sleep can help, but it's a not a whole cure for depression.