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 can pray at a gravestone, or possibly ring a bell at one, to cause any of a number of good and bad effects:

• Summon a ghost, which might be hostile or peaceful.
• Summon a hostile undead, flavored as it forcing its way up out of the grave. Usually a zombie.
• Polymorph you into a zombie or mummy of your own race.
• Put you to sleep, ignoring resistance.
• Create a swarm of hostile manes or lemures.
• The headstone falls on your foot, dealing some damage, less if wearing boots (“killed by a falling tombstone” if fatal).
• If the grave is a bones pile grave, the ghost of the player always appears. It will be peaceful if their alignment matched your own and hostile otherwise.
• Various cosmetic messages that do nothing. “The air suddenly turns cold.” “The engraving on the headstone shimmers for a moment.” “A shiver runs down your spine.”
• The dirt cracks open and spits out any items buried under it.
• A voice from nowhere whispers a random rumor, true or false.
• The epitaph on the grave changes. This does not flag the grave.
• Get an alignment bonus for honoring the dead (this is only if triggered by praying). “You feel that the one buried here is content.” Bonus might be larger if you’re lawful or a Priest, probably 1 otherwise. This should possibly happen before and independently of any other effects.
• Limited identify of 1 item.
• Type-identify of multiple un-type-identified items in the inventory, but the player doesn’t get to pick which. Might pick 0 items (flavored as the spirit not knowing anything new about what the hero is carrying).
• Enlightenment.
• Temporary telepathy / sleep resistance / infravision / other intrinsic.
• The gravestone rumbles and moves aside, revealing a branch staircase to a just-created-now one-level branch, a small one-off “crypt” floating branch that contains some undead, some boxes containing corpses, and some treasure. This only happens on gravestones in the Dungeons of Doom.
• You “feel that the dead are restless”, aggravating monsters level-wide. All gravestones on the level become flagged against further attempts. This effect could be added to and independent of other ones, and if so, it’s a chance dependent on the number of already flagged gravestones on the level, which provides a nice way to balance it on levels that have massive amounts of gravestones.

After you get one effect, the gravestone is flagged so that it doesn’t happen again (or, more evilly, that any further attempts will only result in bad effects). Possibly shouldn’t work at all on graveyard levels, because they have too many graves to balance these effects.

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.