There are two good reasons why it is more efficient to water the garden in the morning.
First, plants have small pores called stomata through which they draw water and the stomata close at night and open in the water. When you water in the morning it enables the plant, through the open stomata, to draw in water and if the plant is hydrated, it will withstand a hot sunny day much better. The second reason is that water evaporates less in the cooler morning that later in the day when temperature rise, providing more for the plant.
Watering at night leaves the water in the soil, because the plant's stomata are closed, so the plants cannot take up water and it also creates a more comfortable environment for slugs.