I am a unchained summoner who has been allowed to take the Synthesist archetype from summoner. I took the Inevitable sub-type.
At 4th level I become immune to fatigue and exhaustion. At 12th level I become immune to sleep. With being immune to fatigue, exhaustion, and sleep, while fused/melded with the Eidolon, do I need to "sleep" (as in pass out and not pay attention to the world)?
I understand that I need to rest for 8 hours still to prepare my unchained summoner spells, but could I keep watch for the entire night and thus give the rest of the party their sleep.
The rules for rest are located here (some scrolling required).
Best Answer
They won't need sleep, but they will need some form of rest to regain daily spells.
While fused with their eidolon, a Synthesist summoner gains the benefits of its evolutions:
Because of the wording of "base evolutions" in the unchained eidolon's features, their progressive upgrades also qualify as evolutions. The 4th-level unchained eidolon of the Inevitable subtype gains an evolution that grants immunity to fatigue and exhaustion. And at 12th level, they gain immunity to sleep. Not just magic sleep.
So the synthesist (while merged with their eidolon) can skip sleep without becoming fatigued. And at level 12, they gain sleep immunity, and can't be put to sleep.
However, this does not affect the rest time required for the purpose of reading their daily spells. Whether or not a spontaneous caster is fatigued or exhausted, they still require rest and concentration in order to get their spells ready.
The rules for arcane spellcasting were originally published in the Core Rulebook, when the only arcane spellcasters were sorcerers and bards. But these restrictions also apply to other arcane spellcasters, which includes the unchained summoner. The quoted rules below should therefore apply to summoners.
Following the "(just like a wizard)" reference, we learn what rest means:
In other words, the summoner could use up their spells, then stay awake all night, concentrate on being on lookout, and then feel fine the next morning without fatigue or exhaustion. But if they put effort into serving lookout for 8 hours (such as perception skill use to detect threats), then they won't have any of their spells ready.
Regarding the rules for resting on PFSRD:
The PFSRD mentions some non-core rules about accumulating penalties versus sleep effects. It fails to mention that these are optional. These variant rules were mentioned once in a part of Trial of the Beast, which is book 2 of the Carrion Crown adventure path. On page 17, it mentions:
These variant rules were designed as an optional plot device to keep the players moving, because they are at a point in the Carrion Crown story where timing is important. But more importantly, while the synthesist is immune to sleep, these penalties would not affect them anyway. The monster rules that define "immunity" include the following:
So if something would cause the synthesist to take a penalty versus sleep effects, the synthesist would ignore it while they have their eidolon's sleep immunity.