Sleepless PCs and Exhaustion Rules

Question

Xanathar's Guide to Everything gives the optional rule for "Going without a Long Rest" (emphasis added):

Going without a Long Rest.
A long rest is never mandatory, but going without sleep does have its consequences. If you want to account for the effects of sleep deprivation on characters and creatures, use these rules.
Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion.
It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.

And the PHB specifies a Long Rest as:

Long Rest
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

If a character has a feature such that they don't need to sleep, do these rules affect them? Would they still have to stave off exhaustion (and make those Constitution saving throws) if they don't sleep?

Answer 1

If you don't need to sleep, you don't suffer exhaustion from going without sleep.

Many of the rules do not account for the few exceptional characters that can avoid sleeping. The closest things that come to my mind to make a ruling about it are the aspect of the moon warlock invocation (Xanathar's Guide to Everything) and the undying nature Undying warlock feature (Sword Coast Adventurer's Guide).
Both these features allow a character to not need to sleep. The first one states clearly that you can spend all 8 hours of a long rest doing light activity, whilst the second one simply states that you still require rest to reduce exhaustion and still benefit from finishing short and long rests.

By reading these features, we can assume that a character that does not require sleep can safely take a long rest substituting the 6 hours of sleep with light activity.

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This assumption is based on the rule Specific Beats General (Player's Handbook page 7):

"If a specific rule contradicts a general rule, the specific rule wins."

I also take into account what is stated at the bottom of the first paragraph of Chapter 2 of Xanathar's Guide to Everything:

"The material in this chapter is meant to make your life easier. Ignore anything you find here that doesn't help you, and don't hesitate to customize the things that you do use. The game's rules exist to serve you and the games you run. As always, make them your own."