Sleepless PCs and Exhaustion Rules

The rules specifically speak about the need to sleep. If a PC or any character for whatever reason does not sleep, by RAW, why would this rule effect them?

Xanathar’s Guide to Everything gives the optional rule for "Going without a Long Rest"

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 the rest is interrupted by a period of strenuous activity-at least 1 hour of walking, fighting, casting spells, or similar adventuring activity-the characters must begin the rest again to gain any benefit from it. At the end of a long rest, a character regains all lost hit points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a long rest. A character can’t benefit from more than one long rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.

If a character des not need to sleep, do these rules effect them?

I understand that sleep is not rest. However, both of these rules specifically state sleeping is a requirement. The question was not is sleep the same as rest.

How dangerous is this modified exhaustion compared to other harmful conditions?

In this question, I asked about the ramifications of monsters causing exhaustion:

How dangerous is exhaustion?

The answers concluded, that this would generally be very risky and dangerous.

I therefore decided to limit myself to a reduced threat version for the time being, which has the following modifications:

1. Limited effect

A target only gains exhaustion levels this way up to three levels. Any excess levels that would be caused this way are not applied.

And

2. Easy recovery

Taking a short rest or casting lesser restoration removes one level of exhaustion gained this way. Taking a long rest or casting greater restoration removes all levels of exhaustion gained this way.

How does this compare to other harmful conditions, such as stunned or paralyzed? Could I replace a stun or paralyze effect with one level of this reduced threat exhaustion and maintain a similar power level of the monster?

How dangerous is exhaustion?

I’m interested in adding exhaustion effects to monsters.

Related questions:

There is this question about causing exhaustion: Are there any ways to force levels of exhaustion onto another creature?

There is question about removing it: Ways to remove exhaustion

And this question about making a spell causing it: What level should this exhaustion-causing spell be?

Considerations

  • When looking at exhaustion (mostly from a combat perspective), the first two levels of exhaustion seem rather benign. The 3rd and 4th have a high impact on combat performance. By 5th level one is basically done for.

  • The linked question above about removing exhaustion shows that this is a hassle.

  • There are apparently no official monsters with such an effect. Though, there is the accursed defiler from Kobold Press’s Tome of Beasts which is not official but at least playtested.

Question

How can I account for an exhaustion effect when balancing a monster? Assume a monster with one attack per turn which after dealing damage asks for a saving throw to avoid one level of exhaustion.

I assume this would have a high impact, so I would also consider adding it as an x/day attack instead or capping the exhaustion stacking at level 3 or 4.

If you have good reasons to say this is a bad idea, please tell me as well.

Does a rest remove ALL chase-induced exhaustion or just ONE level?

Most exhaustion can be removed by a long rest, food, and drink (PHB 251)

Finishing a long rest reduces a creature’s exhaustion level by 1, provided that creature has also ingested some food and drink.

Exhaustion caused by dashing while in a chase, however, is removed by a short OR long rest, with nothing else required. (DMG 252)

A creature can remove the levels of exhaustion it gained during the chase by finishing a short or long rest.

I understand that a short rest alone can remove chase exhaustion.

What I don’t understand is whether one rest removes multiple levels of exhaustion or just one. That is, is the second quote above more explicitly written as:

  1. A creature can remove ALL the LEVELS of exhaustion it gained during the chase by finishing a SINGLE short or long rest.

(or)

  1. A creature can remove ONE of the LEVELS of exhaustion it gained during the chase for EACH short or long rest that it finishes.

What level of exhaustion will a Revivified creature have?

The Revivify spell description says:

You touch a creature that has died within the last minute. That creature returns to life with 1 hit point. This spell can’t return to life a creature that has died of old age, nor can it restore any missing body parts.

If the creature died from 6th level of exhaustion, can it be returned to life with Revivify? What level of exhaustion will it have after revival?

What’s the flying speed of someone under the Fly spell with two levels of exhaustion?

Recently, our party barbarian gained two levels of Exhaustion. As per the rules of that condition (PHB, p. 291), the effect of level 2 Exhaustion is:

Speed halved

The fly spell (PHB, p. 243) has the following effect:

You touch a willing creature. The target gains a flying speed of 60 feet for the duration.

We wondered what the barbarian’s speed would have been if fly was cast on him. Which of the above rules would be considered more specific? Would the barbarian be able to fly for 60 feet, or only 30 feet, per turn?