I'm more inclined to agree with SingerOfTheFall's answer.
Upon Gaara's birth (Shipuuden episode 297 and Naruto chapter 547, page 4), he already had his dark rings around the eyes.
I know he already had the Shukaku sealed inside him, but I find it a bit odd that he was born suffering from insomnia.
Plus, the translation I read of chapter 135 does not refer to the dark rings, but rather to the 'bags under his eyes' (although this could be a problem with the translation...):
Furthermore, as SingerOfTheFall said, there are several other Suna shinobi that have dark rings around their eyes, including Gaara's father when he uses his Gold Dust:
http://i.imgur.com/9i6G1oO.jpgOn a side note, I think it can also be the influence of the tanuki (the 'inspiration' for the Shukaku) in the character conception, since the tanuki too has dark rings around his eyes.
http://i.imgur.com/ihJX349.jpgSo this trait could be somewhat similar to the whiskers Naruto has from the Kyuubi. He had these whiskers even prior to being the Kyuubi's host:
http://i.imgur.com/f4dOcIE.pngThis could mean that the dark circles are a character trait (conceived upon character design) that were a result of the Ichibi being sealed in Gaara even before his birth, thus being present even after the Bijuu's extraction.