What can Diurnal Sect do for you?
Diurnal, acquired at level 30 through the Astrologian quest line makes it so that Aspected Benefic (lvl 34) and Aspected Helios (lvl 42) will apply their regular heal while leaving behind a Heal Over Time (HoT). This is a buff to the target(s) that tics every few seconds, healing a small portion of the players HP. This is similar to Regen and Medica II on White Mage.
Diurnal also lowers the amount of enmity you generate while increasing your attack speed by 5%, so if you're going to be DPSing, this is the Sect for you!
What can Nocturnal Sect do for you?
Nocturnal, acquired at level 50 through the Astrologian quest line makes it so that Aspected Benefic (lvl 34) and Aspected Helios (lvl 42) will apply their regular heal while leaving behind a shield on the target player(s). The shield that you applied to a player with Aspected Helios will be equal to the amount of HP you healed. For example, if Aspected Helios a target for 100 HP, the shield will also absorb the next 100 points of damage done to that player, and any damage over 100 points will hit the player (a player hit for 200 damage would only lose 100 hp). Aspected Benefic however shields for 130% of the heal, so a heal for 100 would shield for 130 damage. Nocturnal shields do not give the same benefit as Scholar shields when they Crit.
Nocturnal also lowers any enmity you generate and offers a 5% potency increase to your heals.
How to use your Sects?
When you activate a Sect it stays on permanently unless you turn it off, change classes, or activate the other Sect—you can only have Diurnal OR Nocturnal active, not both. Additionally, you cannot change sects after entering a battle, so you must decide which Sect to stick with BEFORE you or your party engages enemies. Once all enemies are defeated you can change Sects if you choose.
Which Sect is right for you?
Both Sects have their merits, however there are times where you must pick one according to your co-healer in 8 man content, so let's start there!
Nocturnal Shields DO NOT stack with other AST Nocturnal shields and they DO NOT stack with Scholar shields. However they DO stack with Diurnal HoTs and White Mage HoTs.
Diurnal HoTs DO NOT stack with the HoTs of another Diurnal ASTs. However they DO stack with White Mage HoTs, Scholar shields and Nocturnal shields.
Therefore; when playing alongside a scholar you must use your Diurnal Sect. When playing alongside a White Mage, you can use any stance. When playing alongside an Astrologian, one of you must be in Diurnal and the other must be in Nocturnal. Failing to follow this guideline will result in a loss of healing, wasted MP by both healers, and confusion. Also you will be removed from my academy of AST Sects.
Now let's look at personal preference. When healing alone or with a White Mage you get to choose your Sect. Many will argue that when you're with a White Mage you should always play in Nocturnal, but I say this is hardcore bad advice. It's true that there's a certain synergy to shields and regens, but in some fights the value of shields really shines through, whereas in other fights you'll find that HoTs are just far superior. For example, against Cuchulainn (big ugly guy that vomits on you) In Void Arkk there's a lot of repetitive damage and bleed debuffs. In a situation like this, Diurnal is FAR superior with its HoTs compared to Nocturnal shields which would need to be constantly reapplied. Any fight where you feel the damage is repetitive and unpredictable, Diurnal is the power-Sect.
Now let's look at a different sort of fight—Ravana EX. In this fight everything is highly predictable with regular breaks in damage. Knowing the dance and being able to predict the damage makes Nocturnal a very powerful Sect. In this case shields will absorb the blunt of damage for all major hits, and with the increased healing potency from this Sect you can quickly and easily top off any players. Also, the ability to instantly apply a powerful shield to a player allows you to compensate for a lot of individual slip ups by party members if you have the awareness to see what people are doing. The misconception that White Mage works best with Noct AST comes from the fact that Scholar and White Mage work so well together, however Noct AST does not fulfill the role of Scholar, they are two very different jobs. A Scholar has access to a party-wide regen and fast/repetitive single target heals as needed, and excels at sticking to Cleric Stance for DPS, whereas AST offers party support and a completely different utility belt.
At the end of the day you can manage any fight in either Sect, so it's really a matter of preference, but IMO there are circumstances where one Sect really outshines the other in the hands of an experienced healer.
In the case of being a lone healer (4-man content) I recommend Diurnal for regular trash pulls, since those involve consistent/repetitive damage for the most part. On certain bosses, such as Sohm Als first boss, I've found that Nocturnal is the ideal stance from a healer perspective, however I find this content easy enough that I typically stick to Diurnal for its DPS increase. And the same argument can be made for the aforementioned Ravana EX...
And I'll wrap this up by saying this; I'm a big fan of the Nocturnal stance and its play style, however you'll find that the majority of content is best tackled in Diurnal, unfortunately. Affective use of shields requires more anticipation of incoming damage compared to HoTs, meaning HoTs are better on the learning curves as well as any other situation where damage is unpredictable and/or repetitive. Simply using DF to pick up strangers can often cause enough unpredictability to justify using Diurnal over Nocturnal, as well as the simple fact that Diurnal is more MP friendly. Additionally, because any fight can be handled in either stance it just makes sense to use Diurnal just so you can DPS a little faster/longer when appropriate. I use both stances, but it's rare that Noct is actually ideal if you look at the big picture.
Ty for letting me share AST Sects with you.
A virgins guide to AST Sects
A summary of Astrologian Sects and how to use them.