Steed in nightwear
What is the perfect gentleman John Steed wearing when he is asleep?
Two times during The Avengers one gets a look inside Steed's bedroom and a few times a look at his pyjamas and dressing gowns.

Episodes which see Steed's nightwear are:

Death Dispatch, Mr Teddy Bear, Intercrime and The White Dwarf in series 2, Death of a Batman from series 3,  Too Many Christmas Trees in series 4 and a last time in series 5 episode Dead Men's Treasure.
A fully dressed Steed is wearing a.towelling dressing gown over his pants, shirt and cummberbund in Death Dispatch, while Steed is stripping down to his underwear in Mr Teddy Bear when the squad is looking for poison residues on his clothes. The towelling dressing gown, which Steed gets obviously belongs the members of the squad's office.
Intercrime.offers the first glimpse on Steed's pyjamas
Steed hopes to get some information from one of the Intercrime criminals. Therefore he takes care of the man, who has been shot in the head and Steed is pretending to be a member of the organisation. The man is lying in Steed's bed and wearing one of Steed's pyjamas, that has an unusual pattern (not a Paisley pattern) and seems to be very colourful.

The picture also confirms that Steed allows smoking in his bedroom what matches the ashtray in his series 4 episode Too Many Christmas Trees.
Steed wearing a pyjama can be seen for the first time in episode The White Dwarf. When Cathy Gale arrives at his flat 30 minutes after he has called her, he did not yet had time to chance due to other phonecalls he made.

He's wearing a long sleeve, light colour revere collar pyjama made of cotton (?), and open leather slippers.
His lightweight shawl collar dressing gown is fastened with a tasselled belt, it has no breast pocket and no visible side pockets. The colour is dark, maybe blue or burgundy with a small medaillon design.
He's wearing the same dressing gown over a dark polo shirt in The Gilded Cage and in Death of a Batman. In that episode he's only wearing the pyjama bottoms (or already his pants) and no top.
Another sleepwear add for Steed's wardrobe.can be seen in Too Many Christmas Trees. It seems to be a dark pure wool or cashmere men's smoking jacket with two open side pockets.
He's wearing a light colour (cream or sky?) satin or poplin button up pyjama with a revere collar and no breast pocket, the pyjama bottoms have a drawstring tie and he wears full slippers.
In Series 5 episode Dead Man's Treasure Steed is waiting for a colleague at 3 am when Mrs Peel, who's returning from a party, arrives in the middle of the night at his flat with a bottle of Champagne.
Steed is wearing a violet lightweight dressing gown (silk?) with black polka dots. This is the last time one sees Steed in any form of nightwear during The Avengers.
Sesason 4: Too Many Christmas Trees
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Series 3: The White Dwarf
Series 3: Death of a Batman
Series 2: Intercrime
Series 2: Death Dispatch
John Steed's Fashion