diff --git a/backend/services/notification/__tests__/preferenceService.test.ts b/backend/services/notification/__tests__/preferenceService.test.ts
new file mode 100644
index 00000000..d49622d9
--- /dev/null
+++ b/backend/services/notification/__tests__/preferenceService.test.ts
@@ -0,0 +1,43 @@
+import { describe, expect, it, beforeEach, afterEach, jest } from '@jest/globals';
+import { NotificationPreferenceService } from '../preferenceService';
+
+describe('NotificationPreferenceService', () => {
+  let service: NotificationPreferenceService;
+
+  beforeEach(() => {
+    service = new NotificationPreferenceService();
+  });
+
+  afterEach(() => {
+    jest.useRealTimers();
+  });
+
+  it('returns updated preferences and merges nested patches', async () => {
+    expect(await service.getPreferences('user-1')).toBeNull();
+
+    await service.updatePreferences('user-1', {
+      channels: { email: false } as never,
+      quietHours: { enabled: true } as never,
+    });
+
+    await expect(service.getPreferences('user-1')).resolves.toEqual({
+      userId: 'user-1',
+      channels: { push: true, email: false, sms: false, inApp: true },
+      frequency: 'immediate',
+      quietHours: { enabled: true, startTime: '22:00', endTime: '08:00', timezone: 'UTC' },
+    });
+  });
+
+  it('evaluates quiet hours in the configured timezone', () => {
+    jest.useFakeTimers().setSystemTime(new Date('2026-03-01T23:30:00.000Z'));
+    const preferences = {
+      userId: 'user-1',
+      channels: { push: true, email: true, sms: false, inApp: true },
+      frequency: 'immediate' as const,
+      quietHours: { enabled: true, startTime: '22:00', endTime: '08:00', timezone: 'UTC' },
+    };
+
+    expect(service.shouldDeliverNow(preferences)).toBe(false);
+    expect(service.shouldDeliverNow({ ...preferences, quietHours: { ...preferences.quietHours, enabled: false } })).toBe(true);
+  });
+});
diff --git a/backend/services/notification/preferenceService.ts b/backend/services/notification/preferenceService.ts
index 7eef91e0..4991bd57 100644
--- a/backend/services/notification/preferenceService.ts
+++ b/backend/services/notification/preferenceService.ts
@@ -1,3 +1,5 @@
+import { logger } from '../logging';
+
 export interface NotificationPreferences {
   userId: string;
   channels: {
@@ -15,25 +17,65 @@ export interface NotificationPreferences {
   };
 }
 
-import { logger } from '../logging';
+const DEFAULT_PREFERENCES: Omit<NotificationPreferences, 'userId'> = {
+  channels: { push: true, email: true, sms: false, inApp: true },
+  frequency: 'immediate',
+  quietHours: { enabled: false, startTime: '22:00', endTime: '08:00', timezone: 'UTC' },
+};
+
+const parseTime = (value: string): number | null => {
+  const match = /^(\d{2}):(\d{2})$/.exec(value);
+  if (!match) return null;
+  const hours = Number(match[1]);
+  const minutes = Number(match[2]);
+  return hours < 24 && minutes < 60 ? hours * 60 + minutes : null;
+};
 
 export class NotificationPreferenceService {
+  private readonly preferences = new Map<string, NotificationPreferences>();
+
   async getPreferences(userId: string): Promise<NotificationPreferences | null> {
-    // Mock database fetch
-    return null;
+    return this.preferences.get(userId) ?? null;
   }
 
   async updatePreferences(userId: string, prefs: Partial<NotificationPreferences>): Promise<boolean> {
-    // Cross-device synchronization logic
-    logger.info('Updated notification preferences for user', { userId, prefs });
+    const current = this.preferences.get(userId) ?? { userId, ...DEFAULT_PREFERENCES };
+    const updated: NotificationPreferences = {
+      ...current,
+      ...prefs,
+      userId,
+      channels: { ...current.channels, ...prefs.channels },
+      quietHours: { ...current.quietHours, ...prefs.quietHours },
+    };
+    this.preferences.set(userId, updated);
+    logger.info('Updated notification preferences for user', { userId, prefs: updated });
     return true;
   }
 
   shouldDeliverNow(prefs: NotificationPreferences): boolean {
     if (!prefs.quietHours.enabled) return true;
-    
-    // Evaluate timezone-aware quiet hours
-    // (Mock implementation)
-    return true;
+
+    const start = parseTime(prefs.quietHours.startTime);
+    const end = parseTime(prefs.quietHours.endTime);
+    if (start === null || end === null) return true;
+
+    let current: number;
+    try {
+      const parts = new Intl.DateTimeFormat('en-US', {
+        timeZone: prefs.quietHours.timezone,
+        hour: '2-digit',
+        minute: '2-digit',
+        hour12: false,
+      }).formatToParts(new Date());
+      const hour = Number(parts.find((part) => part.type === 'hour')?.value);
+      const minute = Number(parts.find((part) => part.type === 'minute')?.value);
+      current = hour * 60 + minute;
+    } catch {
+      return true;
+    }
+
+    return start === end ? false : start < end
+      ? current < start || current >= end
+      : current < start && current >= end;
   }
 }