Add two new elemental planes, those of Positive Energy and Negative Energy.

The Plane of Positive Energy:

• The full level is lit.
• Has lots of yellow lights, trees, and whatever plant-based monsters exist in the game.
• Everything is treated as if it has regeneration. The player can regenerate current HP past its maximum, but if this gets too far past the maximum, they instantly die from being unable to contain the energy (there are warning messages to this effect as you approach this point).
• Items in the player’s and monsters’ inventories can randomly get blessed, enchanted, or charged - even if this runs the risk of overenchanting and destroying them.
• Eating any food restores HP as well as nutrition.
• All exercisable attributes are exercised at regular intervals.
• All sleep effects are negated.

The Plane of Negative Energy:

• The full level is dark.
• Contains lots of undead.
• Everything constantly loses hit points. If you have regeneration, it either nullifies or only partly nullifies this effect.
• Items in the player’s and monsters’ inventories can randomly get cursed or lose enchantment or charges (going negative on things which are capable of being negative).
• You lose several times as much nutrition per turn as normal.
• All abusable attributes are abused at regular intervals.

Make shopkeepers more susceptible to being paralyzed or put to sleep than they currently are, so it is easier to directly but non-lethally steal from them.

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

Spellcasting monsters, at least the mid to upper range ones, should be able to cast sleeping at the player.

Artifact blindfold Mask of Hypnos: provides sleep resistance and detection of sleeping monsters when worn, and can be invoked for a sleep ray.

If you feel worried about your pet or sad for a moment over their death while sleeping, you wake up immediately.

Thrown sleeping potions should knock a non-sleep-resistant target out for more than only 0-1 turns.

Remove sleep as a ray. Sleep wands (and monsters with sleep breath like orange dragons and Nazgul) instead shoot a beam of sleep. The sleep spell either also shoots a beam of sleep or causes sleep in an area of effect. Sleep could also possibly be a cone, spreading out in the direction it’s shot in, but cone spells remain unimplemented.

A trap or monster (or possibly change sleeping gas traps or homunculi to this) that causes “sleeping poison”: it starts a very short intrinsic restful sleep timeout, so you don’t fall asleep immediately but have a little time to react.

If you fall asleep while having astral vision, you get astral projection. This consists of you spawning as some sort of incorporeal monster which can phase next to your body. Your body can’t move. Your projection can use your items like normal (consumables and charges are used up). If your projection dies, you return to your body and sleep out the remaining duration. If you wake up, your projection is destroyed and you return to your body normally; however, combat will not awaken you. If something else kills your body while you’re projected, you die and the game ends.

Being attacked while asleep doesn’t have a random chance of awakening you, but instead it reduces your remaining sleep time by some amount.

When you fall asleep, you take d3 damage from falling to the floor. Message for this: “You tumble to the floor”.

If a monster steps on a squeaky board near you when you are asleep, it may wake you up. (Really, this should be the case for any effect capable of waking monsters.)

Dart traps can make you fall asleep, go blind, hallucinate, get confused, etc. This would mesh well with #1922.

Make the sleep spell scale like it does in D&D: you can only put a total of (your level) HD of monsters to sleep.

The amulet of restful sleep confers hungerless regeneration whenever the player is asleep.

Premonition system: under various conditions (crystal balls, clairvoyance, dreaming while you sleep, the Oracle) you can receive a premonition of something that will appear later in your game (this could be almost anything: a monster, object, dungeon feature, special room, etc) which then has a very high chance of generating subsequently.

You get petrified when you faint or otherwise fall asleep while wielding a cockatrice corpse.

Dreaming about more than just noises that are really happening: stuff like fantasy dreams where you dream that you ascend to the status of Demigoddess, etc.

Fumble boots cause you to toss and turn in your sleep if you are sleeping, making you wake up sooner.