Here's my solution for the BetterRest challenge. The prediction for the bedtime is moved into a computed property.

I didn't investigate yet, but wondering why we need to create an instance of the ML model every time we want to make a prediction. Probably it's lightweight enough -- for sure for this app.

import CoreML
import SwiftUI

struct ContentView: View {

    @State private var wakeUp = defaultWakeTime
    @State private var sleepAmount = 8.0
    @State private var coffeeAmount = 1

    static var defaultWakeTime: Date {
        var components = DateComponents()
        components.hour = 7
        components.minute = 0
        return Calendar.current.date(from: components) ?? Date.now
    }

    var predictedBedtime: String {
        var bedtime = ""
        do {
            let config = MLModelConfiguration()
            let model = try SleepCalculator(configuration: config)

            let components = Calendar.current.dateComponents([.hour, .minute], from: wakeUp)
            let hour = (components.hour ?? 0) * 60 * 60
            let minute = (components.minute ?? 0) * 60

            let prediction = try model.prediction(wake: Double(hour + minute), estimatedSleep: sleepAmount, coffee: Double(coffeeAmount))

            let sleepTime = wakeUp - prediction.actualSleep

            bedtime = sleepTime.formatted(date: .omitted, time: .shortened)
        } catch {
            bedtime = "Sorry, there was a problem calculating your bedtime."
        }
        return bedtime
    }

    var body: some View {
        NavigationView {
            Form {

                Section("When do you want to wake up?") {
                    DatePicker("Please enter a time", selection: $wakeUp, displayedComponents: .hourAndMinute)
                        .labelsHidden()
                }

                Section("When do you want to wake up?") {
                    DatePicker("Please enter a time", selection: $wakeUp, displayedComponents: .hourAndMinute)
                        .labelsHidden()
                }

                Section("Desired amount of sleep") {
                    Stepper("\(sleepAmount.formatted()) hours", value: $sleepAmount, in: 4...12, step: 0.25)
                }

                Picker("Daily coffee intake", selection: $coffeeAmount) {
                    ForEach(1..<21) {
                        Text($0 == 1 ? "1 cup" : "\($0) cups")
                    }
                }

                Section("Your ideal bedtime") {
                    Text(predictedBedtime)
                }
                .headerProminence(.increased)
            }
            .navigationTitle("BetterRest")
        }
    }
}