Hey kalamamama,
While we wouldn't know what specifically caused the issue on this particular night given the information you've provided to this point, we'd suggest making sure you have a sleep schedule set up to automatically enable Sleep focus. It's certainly possible that since it was in Do Not Disturb instead of Sleep focus this might have impacted things.
Set up Sleep
Getting enough sleep is important to your overall health. The Health app on iPhone can help you set a sleep goal, and track your progress over time in meeting that goal.
1. Open the Health app on your iPhone.
2. Tap Get Started under Set Up Sleep1, then tap Next.
3. Follow the onscreen prompts to establish:
• Sleep Goals: Set the number of hours you'd like to spend asleep.
• Bedtime and Wake Up times: Set when you'd like to go to bed and wake up.
• Sleep Focus: To reduce distractions, iPhone can simplify your Lock Screen and turn on Sleep Focus at your scheduled bedtime. You can also give certain people or apps permission to notify you. Learn how to customize a Focus.
•Track Sleep with Apple Watch: Wear your watch to bed to track your sleep. This option is available during setup if you've paired your Apple Watch with your iPhone beforehand. If you pair your Apple Watch after setup, you can still turn on Track Sleep with Apple Watch later.
Additionally, to get more accurate recordings, try out the following:
Get more accurate results with your Apple Watch
If you paired your Apple Watch after completing Sleep setup, you can still turn on Track Sleep with Apple Watch. In the Watch app on your iPhone, tap the My Watch tab, then tap Sleep. Then tap Track Sleep with Apple Watch to turn on this setting. To get the most accurate results when wearing your watch to sleep:
• Turn on Charging Reminders. On your iPhone, open the Watch app and tap the My Watch tab, then tap Sleep. Tap Charging Reminders to turn on a reminder on your Apple Watch to charge before your Wind Down time. If your watch battery dies, it won't track your sleep data.
• Make sure that you're wearing your Apple Watch at a comfortable fit. If your watch is too loose, the accelerometer might register too much movement during your natural sleep.
We hope this information helps out. It's available in more detail here: Track your sleep on Apple Watch and use Sleep on iPhone - Apple Support
Thanks for using Apple Support Communities.
Take care!