Longer duration: 30 sec?

Cancelable at any time, but waking up will take time depending on the PCs listen skill. Something like roll against PCs listen skill with DC of 25 and amount failed causing 1 second delay for player. This waking up stuff cannot take longer than the original rest duration (i.e. if you cancel it after 29 seconds, the maximum delay will be 1 second).

If no camp set then always ask for verification whether you want to sleep. If camp set, then no verification.

Screen get dark/darker when you rest.

During rest each PCs survival skill gets additional -10 modifier (reduces the survival range).

If player is attacked during the rest (i.e. not only when there is monsters nearby, but only when they are attacked), they will start waking up automatically. If PCs want to wake up before being attacked they can cancel the rest themselves, but if they decide that attack is not dangerous enough for them to be bothered (i.e simulating that they would wake up, check out situation, and go back to sleep) they can continue sleeping as long as they are not attacked (i.e. getting damage). Getting damage will speed up the waking up, i.e. each hitpoint lost will add 1 to the listen skill.

If spawnpoints nearby then check when they were last triggered by rest system. If less than 120 seconds ago, do nothing. If more than 120 seconds, then spawn monsters at them after 5 seconds of sleep.

Players should be able to rest regardles that there is monsters nearby if they like, i.e they should not get errors complaining that there is monsters nearby (if possible).

Any monster should be able to be set as “guard”, which means it does not attack PCs because of them resting. They will attack if they are close enough to see the PCs normally.

Only monsters should attack directly. Animals very rarely (herbivores never, omnivores with around 10% probability). Intelligent creatures (humanoids etc), should check the combined party strength against their own, and if resting party is much stronger then it should not attack. Something like

(100 - ((levels(party) - levels(attackers)) * 10) %

I.e if the strenght of the party is 10 times more than the attackers strength then they do not attack.

Attackers is calculated as all monsters (or monsters from encounters) nearby that belong to same group.

Setting camp uses the survival skill of the setter. This is used to decrease the probability monsters will attack, i.e. the survival_skill * 5 is decreased from the probability above. I.e. survival skill of 10 will reduce the probability by 50.

Party of 4 persons of levels 4, 4, 5 is going in the goblin valley. Their strenght is 13. There is group of 4 of goblin raiders nearby (XP level 1) and 2 encounter triggers. Those 2 encounter triggers are triggered (after 5 seconds of sleep, but only once per 120 seconds of people sleeping, i.e. if multiple people sleep they are triggered only once). That will create 2 new groups of 4 of goblins. This makes the total goblin count 12, i.e. their strength is 12. As they didn't make camp, they do not get camp bonus, and the probability of the goblins to attack is (100 - (13 - 12) * 10) = 90%. As 12 goblins is considerable threat, probably everybody canceled their sleep and they start fighting and killing those goblins. That fight probably takes more than 120 seconds in total (including timeout after the fight when you cannot rest again yet).

So this time the decide to be smart, they make the camp now. Ranger with skill of 9 makes the camp.

Then they try to sleep again. There is no more goblins nearby, but there is still the 2 encounter triggers, and as there is more than 120 seconds from the last spawn, they will trigger 8 more goblins. Now the base probablity to attack is (100 - (13 - 8) * 10) = 50%. As they set up the camp, and ranger had skill of 9, that will reduce that probablity by 45% i.e. down to 5%. So they now have 95% of possibility to be able to sleep without creatures attacking. Note, that if any of those spawned monster was too close to the group, so they can actually see the group, then those will of course attack, the probability will just limit the those further away will not start attacking.

Party leader can forbid rests without setting up camp?

DM can forbid rests, or also setting up the camp.

DM can set rest to spawn a DM custom encounter.

Specific placable can forbid rests near.