Nepal is a country of extremes in terms of temperature and altitude changes from the north to the south; what you need to bring to Nepal will simply depend on where you intend to go in the country. The northern hills and mountains are not as warm during the day, but if you are hiking, the exercise will keep you warm enough that you can get by with light clothing. South in the Terai plain region of the country, it is very warm during the day but can be somewhat cool at night. Kathmandu is warm during the day, which means light clothing is just fine. At night, however, temperatures can get quite chilly; make sure you pack some warm clothes.
Because of the quick shifts in temperature and altitude in Nepal, it is a good idea to wear layers. This gives you the flexibility you need to stay comfortable. Trekkers should prepare both light and heavy yet comfortable clothing. This applies not only to the day, but also to the nighttime since most of the guesthouses lack heating. Chitwan National Park is filled with thorns, branches, tall grass, bugs, and other forms of nuisances to your feet and exposed skin. It is therefore important to pack some long sleeves, long pants, and lightweight shoes (such as hiking shoes and trail runners) that are comfortable and keep debris out. In addition, it's best to bring a hat and sunglasses to protect yourself from the bright rays.