Why Auth::user() return null from controller method?

I'am develop an admin panel. It's guard by authentification. I want to retrieve user data after login. But controller method return null on Auth::user(). Why can it happened? Thanks in advance!

Controller

namespace MyNamespace\App\Controllers\System;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Auth;
use App\Http\Controllers\Controller;

class UserController extends Controller
{
    public function index (Request $request) {

        return response()->json(Auth::user(), 200);
    }
}

Routes web.php

Route::group(['middleware' => ['web']], function () {

    Route::get('login', 'MyNamespace\App\Controllers\Auth\LoginController@showLoginForm')->name('login');

    Route::prefix('api')->group(function () {

        Route::prefix('system')->group(function () {

            Route::get('user', 'MyNamespace\App\Controllers\System\UserController@index');
        });

        Route::post('login', 'MyNamespace\App\Controllers\Auth\LoginController@login');
        ...
    });
});

config/auth.php

...
'providers' => [
    'users' => [
        'driver' => 'eloquent',
        'model' => MyNamespace\App\Models\User\User::class,
    ],
],

via Chebli Mohamed