diff --git a/README.md b/README.md
index cd31e9e..7c71913 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1685 TILs and counting..._
+_1686 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -702,6 +702,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Clean Up Old Homebrew Files](mac/clean-up-old-homebrew-files.md)
 - [Convert An HEIC Image File To JPG](mac/convert-an-heic-image-file-to-jpg.md)
 - [Default Screenshot Location](mac/default-screenshot-location.md)
+- [Detect How Long A User Has Been Idle](mac/detect-how-long-a-user-has-been-idle.md)
 - [Disable Swipe Navigation For A Specific App](mac/disable-swipe-navigation-for-a-specific-app.md)
 - [Display A Message With Alfred](mac/display-a-message-with-alfred.md)
 - [Find The Process Using A Specific Port](mac/find-the-process-using-a-specific-port.md)
diff --git a/mac/detect-how-long-a-user-has-been-idle.md b/mac/detect-how-long-a-user-has-been-idle.md
new file mode 100644
index 0000000..37648fa
--- /dev/null
+++ b/mac/detect-how-long-a-user-has-been-idle.md
@@ -0,0 +1,39 @@
+# Detect How Long A User Has Been Idle
+
+The `ioreg` utility on MacOS dumps the I/O Kit registry tree. This lets us look
+at the state of all hardware devices and drivers registered with I/O Kit.
+Looking specifically at the Human Interface Device subsystem (`IOHIDSystem`), we
+can find a handful of properties including the `HIDIdleTime`.
+
+```bash
+$ ioreg -c IOHIDSystem | awk '/HIDIdleTime/'
+    | | |   "HIDIdleTime" = 91831000
+```
+
+That value is the number of nanoseconds since a human input device was last
+interacted with. That is the amount of time the user (me) has been idle.
+
+I can convert this to seconds, which is the small amount of time between me
+hitting enter in the terminal and the command finding the idle time.
+
+```bash
+$ ioreg -c IOHIDSystem | awk '/HIDIdleTime/ {printf "%.2f seconds\n", $NF/1000000000}'
+0.13 seconds
+```
+
+I can run this in `watch` to see the elapsed idle time increment.
+
+```bash
+watch -n 1 "echo -n 'Idle time: '; ioreg -c IOHIDSystem | awk '/HIDIdleTime/ {printf \"%.1f seconds\\n\", \$NF/1000000000}'"
+```
+
+After watching the _idle time_ increment for a bit, I can move the mouse and
+watch it reset on the next `watch` loop.
+
+This could be used as part of a script that takes certain actions after the user
+has been idle for a while, like putting the display to sleep or stopping a time
+tracker app.
+
+There is a _lot_ going on in the `ioreg` output and it's hard to make sense of
+hardly any of it. I found running `ioreg -c IOHIDSystem | less`, searching for
+`IdleTime`, and browsing from there to be a good starting point.