It's a REM. Because the acronym REM is usually pronounced as a word that starts with a consonant sound, we use the indefinite article "a".

It's a REM. Because the acronym REM (Rapid Eye Movement—sleep phase) is usually pronounced as a word that starts with a consonant sound, we use the indefinite article "a".

A REM cycle may last up to an hour.

An REM cycle may last up to an hour.

Be aware that we use "a" before words that start with a consonant sound.

A REM sleep is an important phase of sleep in mammals and birds, characterized by random rapid movement of the eyes.

Being woken during a REM period can leave you feeling overly sleepy.

Similarly, when speaking about the random access memory, we say a RAM (not an RAM).

Do you know how to install a RAM disk?