Senior Sleep Cycle

A sub-trope of Grandparental Obliviousness. Basically, if you see a senior citizen in a TV show, most likely, he's either already napping or going to be in a few minutes. And, due to Finagle's Law, it will always be right when he's supposed to be keeping an eye, on something, or about to ... tell

...

you

...

zzz

...

Huh? Wha? Oh, right. There are some elders who can escape this inconvenience. Evil Old Folks tend to be exempt from this rule, as are women in general. Maybe they just drink a lot of coffee. Needless to say, this is an Elderly Trope. It is also more likely to appear in a Comedy Trope.

In real life, several studies have suggested that people actually need less sleep as they grow older. This trope does reflect Truth in Television to some extent, though, as older people often tend to go to sleep earlier than younger adults, and some sleep disorders that can cause daytime dozing, such as sleep apnea and insomnia, are more likely to gradually develop over the course of a lifetime.

Examples of Senior Sleep Cycle include:

Anime and Manga

In Umineko no Naku Koro ni, during a few of the murders, Nanjo's alibi is that he was asleep at the time.
In One Piece, Vice-Admiral Garp is about to punch Luffy when both suddenly fell asleep. Portgaz D. Ace was also prone to this when he's eating.
Carlos of El Dorado V is always asleep when he's on screen, save for two occasions.
Crux from Fairy Tail.

Film

Bernard, one member of the Neighborhood Watch Alliance in Hot Fuzz, spends most of his onscreen time asleep, except for about a minute during the final confrontation.
The wise old man in Labyrinth, much to the chagrin of his hat.

Literature

In Exile's Honor, one of Alberich's undercover identities is an old man who lives this trope. "Falling asleep" after dinner lets Alberich eavesdrop on the upper classes safely.

Live Action TV

"Old Man" of Pawn Stars had been shown to be flat out asleep when he was at work. He is co-owner of the place and so he's allowed to do that.
- Chumlee has rudely lampshaded this fact on more than one occasion. You gotta admire a man who keeps his resumé up to date.
In the Star Trek: The Original Series episode "The Deadly Years", Rapid Aging caused Captain Kirk to doze off in the command chair.

Video Games

The old man Yanni Yogi in Phoenix Wright Ace Attorney kept falling asleep in the middle of conversations, and even the trial, and awakened when his Snot Bubble popped.
Ellen from Harvest Moon 64 has this generally, and it's also notably subverted once, when she dies.
The Old Man at the End of Time in Chrono Trigger (aka Gaspar, the Guru of Time), has almost always fallen asleep against a streetlamp when the heroes visit him, and has to be woken up to start a conversation. Then again, being at the End of Time doesn't usually give him very much to do.

Web Original

The Annoying Orange's Grandpa Lemon. He falls asleep in the middle of conversations and even while being chopped in half.

Western Animation

In Rugrats, whenever Grandpa Lou is put in charge of the babies, he is always asleep within a matter of seconds, a fact that the babies always use to their advantage.
- When he leaves in "Grandpa Moves Out", the family visits him at the retirement home. Tommy and Angelica are watched over by the surrogate grandmother. She shows pictures of her grandchildren while listing the name of the seven dwarfs. When she can't remember the name of the last one, Angelica suggests "Sleepy"; she says she could use a nap and instantly falls asleep.
Abraham J. Simpson is constantly falling asleep, often in the middle of sentences. It's possible he's narcoleptic (especially on the season nine episode "Lisa's Sax" where, after he tried to sing "The Funky Grampa," Grampa ended up falling asleep standing up).
On Arthur, Grandma Thora averts this, but Grandpa Dave plays it straight.
Happens in SpongeBob SquarePants with Mermaid Man.
Professor Farnsworth in Futurama once fell asleep while skiing and woke up at the lodge with a bronze medal around his neck.
Suga Mama on The Proud Family always falls asleep while watching wrestling or when she is supposed to be chaperoning Penny or babysitting the twins.
Parodied on Phineas and Ferb. Linda's mother appears to be succumbing, but she was playfully teasing Lawrence about his boring thimble story.
Cohort of the Ministry of Pain in The Powerpuff Girls in the midst of a bank heist.