Turn the amulet of restful sleep into an amulet of sleep resistance, except it grants restful sleep (and no sleep resistance) instead while it’s cursed, and it still mostly generates cursed.

Artifact ring that provides a cocktail of basic resistances all at once (fire, cold, shock, poison, maybe sleep, maybe acid). This is interesting primarily because it is very useful early in the game and then becomes progressively less useful later on as the hero gets intrinsic resistances.

Could be named “The Ring of Everything Resistance” or “The Chromatic Ring”.

Give Thoth Amon sleep resistance or higher monster MR, because right now it’s too easy to just zap him with sleep and finish him off trivially.

Potion of coffee, a nonmagical potion which confers temporary sleep resistance and, while the sleep resistance is still active, a small bonus towards the odds of learning a spell from a book successfully.

When you are wearing the amulet of restful sleep, you are given sleep resistance whenever the amulet is not causing you to sleep; this is because you shouldn’t be tired in between frequent naps.

Orange mold, a F monster which has a passive sleep attack, and a small chance (equal to the other molds) of conveying sleep resistance.

The blessed potion of gain energy gives temporary sleep resistance.