Sleep Mask

Mom:*wearing mask* What's my present?
Dad: OPEN YOUR EYES!

Mom: *blink* OOH, A blindfold!

—Fairly Oddparents

A mask without eye holes worn when a character is sleeping somewhere with too much light. Often they're seen on planes or trains; oddly enough, though, in fiction, they can appear in perfectly dark private bedrooms.

The Sleep Mask is used to give the audience clues about the personality of the person wearing one. It can signify:

High "class"
Vanity
An effeminate nature

The third is more often used for a man, but can also be used for women. For example, a woman with a Tomboy personality during the day might for contrast wear a sleep mask at night, or one might be used to emphasize one character's girly nature in comparison to another female cast member.

Tropes that may apply: Spoiled Sweet, Spoiled Brat or Rich Bitch. Often but not always worn by the villainess.

Bonus points if the mask has painted eyes.

Examples of Sleep Mask include:

Advertising

One of Toyota's recent Corolla commercials featuring Hatsune Miku has her taking a nap in the said car while wearing a purple sleep mask.

Anime & Manga

The first time Sougo Okita is seen in Gintama, he's wearing one of these with eyes drawn on it, using it to sleep while he's supposed to be on duty. It's become iconic of his character.
Admiral Aokiji wears one at all times as he spends a good deal of his spare time sleeping.
Revolutionary Girl Utena has one complete with eye decorations, worn by a pet monkey of all creatures.

Film

In Matilda, her mom (Mrs Wormwood) a fake blonde who concentrates on her manicure rather than listen to her daughter sleeps with a mask. It has two eyes drawn on it.
Charlotte from The Princess and the Frog.
Albin wears one in the sequel to La Cage aux Folles, La Cage Aux Folles 2.
Used in Thirteen Going On Thirty.
Auntie Mame is wearing one when Patrick bursts into her room to show her his new toy airplane.
In Overboard, the main character's mother is wearing one of these when the yacht is turned around in the middle of the night.
Holly has one in Breakfast at Tiffany's.
Courtney Rawlinson wears one in American Psycho.
In Atlantis: The Lost Empire, Rourke's crew sleep with these on because Lt. Wilhelmina Packard sleeps in the nude and has a tendency to sleepwalk. We don't actually see this of course (thank goodness), rather, Vinny informs Milo of this when handing him a mask.
Claus von Bulow wears one (and maybe earplugs too) in Reversal Of Fortune.

Live Action Television

Worn by Harry Kim in Star Trek: Voyager in the episode "The Cloud". No real reason is given since it is dark outside in outer space, other than to make him look like a naive pillock.
London Tipton uses one of these in The Suite Life of Zack and Cody.
Worn by Lucille Bluth on Arrested Development
In Mash, Charles Emerson Winchester wears one to bed; it's not an unreasonable thing considering the Officer's Tent's flaps are usually up in warm weather and wounded could come any time so they have to get sleep anytime they can off duty.
In The Big Bang Theory, Sheldon wears one in a hotel room (episode "The Love Car Displacement")
Marie Schrader wears one in Breaking Bad.

Toys

All the dolls of the Dead Tired line of Monster High.

Web Comics

Phix of Wapsi Square wears one of these. It is less about representing an aspect of her character, and more about the fact that a sphinx with a sleep mask is hilarious.

Western Animation

Barbie In A Christmas Carol pictured above. Eden Starling is the Gender Flip version of Ebenezer Scrooge in a Charles's Dickens novel adaptation. So, instead of a greedy man, we have a self-centered lady. Not surprising the three Christmas spirits find her asleep with such a mask.
On The Simpsons, Marge's Jerkass sisters Patty and Selma have been seen wearing these while sleeping, as has Reverend Lovejoy.
Jimmy from Ed, Edd 'n' Eddy wears one, at least in the episode "A Glass of Warm Ed".
Heloise on Jimmy Two-Shoes.
Rarity in My Little Pony Friendship Is Magic uses one in Sisterhooves Social.
Linda wears one in Phineas and Ferb, but usually it's to keep her from seeing anything the boys might be up to at night.