I just want to point out though, depression isn't about feeling happy or sad throughout the day. Depression is more of a long-term condition.

It's likely you have something interesting to do or something to keep you occupied in days you're waking up early. You might feel happy those days, but lack of sleep might actually be worsening your depression in the long run. Not saying this is what's happening necessarily, but it's possible.

That said, the article isn't even talking about sleeping less, it's talking about waking up early. But I remember seeing interesting studies that are actually studying the link between sleep deprivation and depression.