Apple Watch Health Features:

Real-time Heart Rate Monitoring: Apple Watch uses sensors to track your heart rate throughout the day and during workouts. You can view your heart rate in the Health app on your iPhone, or on the watch itself.
High & Low Heart Rate Notifications: If your heart rate goes above or below a certain threshold when you're inactive, the watch will alert you.
Irregular Heart Rhythm Notifications: The watch can detect signs of atrial fibrillation (AFib) by checking your heart rhythm and alerting you if it detects irregularities.

ECG App: Available on Apple Watch Series 4 and later, this feature allows you to take an ECG directly from your wrist, which can help detect atrial fibrillation (AFib) and other heart conditions. The results can be shared with a healthcare provider.
The ECG app requires the user to press their finger against the Digital Crown to complete the test, which records your heart's electrical activity.

Blood Oxygen App: Available on Apple Watch Series 6 and later, this feature uses infrared light to measure the oxygen saturation in your blood. It helps give insight into your overall respiratory health, and the watch provides readings of your SpO2 levels (the percentage of oxygen in your blood).
Low blood oxygen levels could indicate underlying health issues like respiratory problems or sleep apnea.

Body Temperature Monitoring: The Apple Watch Series 8 and Apple Watch Ultra (and later) feature a temperature sensor that tracks your skin temperature while you're sleeping. This can help detect changes in body temperature related to various health conditions.
The temperature sensor is particularly useful for tracking menstrual cycles (with the Cycle Tracking app), as changes in body temperature can signal ovulation or other hormonal fluctuations.

Fall Detection: The Apple Watch can detect hard falls using its accelerometer and gyroscope. If a fall is detected, the watch taps you on the wrist and asks if you're okay. If you don't respond, it can automatically call emergency services and send a message to your emergency contacts.
Fall detection is available on the Apple Watch Series 4 and later, and it's turned on by default for people 55 and older (you can also manually enable it).

Sleep App: Apple Watch Series 3 and later include sleep tracking capabilities. The watch can monitor your sleep stages (such as REM, core sleep, and deep sleep) to help you understand your sleep patterns.
It also tracks your sleep duration and gives you insights into how well you're sleeping, helping you improve sleep hygiene.

Move, Exercise, and Stand Goals: The Apple Watch encourages movement with three activity rings:
- Move: Tracks your calories burned through activity.
- Exercise: Monitors minutes of brisk activity.
- Stand: Reminds you to stand up for at least a minute each hour.
It offers customizable goals for each category and provides motivational reminders to help you stay on track throughout the day.

Workout App: Tracks a wide range of workouts such as running, cycling, swimming, hiking, yoga, strength training, and more.
Advanced Metrics for Athletes: The Apple Watch Series 8 and Ultra, in particular, offer more advanced metrics like pace, elevation, cadence, and heart rate zones for athletes.
Swimming Metrics: Apple Watch is water-resistant and tracks swim workouts, including lap count, distance, and average pace.

Mindfulness App: Helps you manage stress by guiding you through breathing exercises (with the Breathe feature) or mindfulness sessions. This helps lower stress and improve mental well-being.
Heart Rate Variability (HRV): The Apple Watch tracks HRV, which can be an indicator of your body's stress levels and overall autonomic nervous system health.

Crash Detection: A feature introduced with the Apple Watch Ultra, Series 8, and later, using an advanced gyroscope and accelerometer to detect severe car crashes. If a crash is detected, it automatically alerts emergency services and contacts.

Cycle Tracking App: The Apple Watch (starting with Series 3) allows users to track their menstrual cycle and symptoms, including fertility windows. The app integrates with the temperature sensor for more detailed cycle analysis and predictions about menstruation, ovulation, and more.
For those using the temperature sensor, Apple Watch provides more accurate cycle predictions by tracking fluctuations in body temperature.

Noise App: Monitors ambient noise levels in your environment and alerts you if the noise exceeds safe levels, which could potentially harm your hearing over time.
Headphone Notifications: The Apple Watch can also monitor the volume levels when you're listening to music or media through headphones and notify you if the sound is too loud and could impact your hearing.

Medications App: The Medications app, introduced with iOS 16, lets you track your medications, vitamins, and supplements directly on your Apple Watch. You can set reminders to take your medications on time and log your doses.

Blood Glucose: While not yet available, there have been rumors and patents suggesting Apple is working on incorporating non-invasive blood glucose monitoring in future models. This would be especially beneficial for people with diabetes to monitor their blood sugar levels easily.

Apple Watch SE (2nd Gen): Offers heart rate monitoring, fall detection, activity tracking, and sleep tracking but lacks ECG, Blood Oxygen, and the temperature sensor.
Apple Watch Series 9: Includes heart rate monitoring, ECG, Blood Oxygen, fall detection, sleep tracking, activity tracking, temperature sensor (for cycle tracking), and more.
Apple Watch Ultra: Includes all of the Series 9 features but adds more advanced features like the dual-frequency GPS, longer battery life, crash detection, and more rugged durability, ideal for extreme athletes and adventurers.