You can help the mind recognize it's night by dimming all the lights down around the time you want him/her to be getting ready for sleep. Close the curtains, turn off TV, turn off screens etc. Make the house quiet and boring - have any other children do very quiet activities or also be getting ready for bed.
Once the baby is asleep you can always have the TV on quietly in another room. Have the same bedtime routine at the same time, including a short warm bath to help sleep. Try and have as structured morning and evening routines as you can (same times waking and eating and getting out into natural light) as these all trigger the brain to expect wakefulness, food, activity and sleep accordingly.
Have everyone talk quietly and calmly as you get into the nighttime routine. Have any boisterous activity stop before entering nighttime routine (i.e don't play with and have them run around in PJS - but warm drink, bath, PJs, book, bed) - remember calm and boring, don't stimulate the mind.
Make sure they are out in natural light asap daily and have plenty of activity during daylight hours.
If you keep all the lights blazing, lots of noise and activity into the evening, together with an eclectic, unpredictable schedule and no daily routine, the brain has no patterns it can learn.
In the summer I would still close all curtains, creating dim light/darkness and quiet and it would help them fall asleep, despite bright sunlight outside.
If necessary have a white noise machine or gentle music to block other sound.