The most common one would probably be

Chill out.

This is both informal and natural. Of course it's often likely to be poorly-received since it's not an instruction that's easy to follow.

It's common enough to have an entry in Merriam-Webster.

That definition provides an alternative:

Calm down.

That one is less informal but still completely natural. However it's not quite as apt if you want someone to lower their vigilance, rather than descend from a state of agitation or anxiety.

Note that technically speaking the quotes in this answer should end in a "!" rather than a "." but I find that doing that in conversational text (e.g. in a chat app) comes off as overly emphatic or imperious.

EDIT:

Another informal expression, which may sound patronising:

Cool your jets.

The most common one would probably be

Chill out.

This is both informal and natural. Of course it's often likely to be poorly-received since it's not an instruction that's easy to follow.

It's common enough to have an entry in Merriam-Webster.

That definition provides an alternative:

Calm down.

That one is less informal but still completely natural. However it's not quite as apt if you want someone to lower their vigilance, rather than descend from a state of agitation or anxiety.

Note that the quotes in this answer can (edit: I incorrectly wrote should) end in a "!" rather than a "." but I find that doing that in conversational text (e.g. in a chat app) comes off as overly emphatic or imperious.

EDIT:

Another informal expression, which may sound patronising:

Cool your jets.

The most common one would probably be

Chill out.

This is both informal and natural. Of course it's often likely to be poorly-received since it's not an instruction that's easy to follow.

It's common enough to have an entry in Merriam-Webster.

That definition provides an alternative:

Calm down.

That one is less informal but still completely natural. However it's less apt if you want someone to lower their vigilance, rather than descend from a state of agitation.

Not that technically speaking the quotes in this answer should end in a "!" rather than a "." but I find that doing that in written text (e.g. in a chat app) comes off as overly emphatic or imperious.

The most common one would probably be

Chill out.

This is both informal and natural. Of course it's often likely to be poorly-received since it's not an instruction that's easy to follow.

It's common enough to have an entry in Merriam-Webster.

That definition provides an alternative:

Calm down.

That one is less informal but still completely natural. However it's not quite as apt if you want someone to lower their vigilance, rather than descend from a state of agitation or anxiety.

Note that technically speaking the quotes in this answer should end in a "!" rather than a "." but I find that doing that in conversational text (e.g. in a chat app) comes off as overly emphatic or imperious.

EDIT:

Another informal expression, which may sound patronising:

Cool your jets.