On warm nights such as we are now having, ventilation is essential. If your windows have trickle vents, these should always be left open.
If you want more airflow, but are worried about leaving your windows open, an ideal solution is to lock them into the night vent position. This is the second position that your windows will open to and can be securely locked in this part open position without any danger from intruders. In windows which have no trickle vents at all, it is ideal for general ventilation.