On your first day in Cuba, take it easy. Take time to acclimatise. Adapting to the climate and culture can be quite difficult. Get used to the country at your leisure. Just relax and soak up your new surroundings.
Some additional recommendations for an enjoyable stay
Allow your body and mind some time to adapt to the new time zone and location. Avoid stress, do not plan to do too much at once. Take it easy on the first day at least. Getting up early is a good habit to get into because the morning temperature is often the most agreeable in hot climates. Besides, the people in the countries you visit are also used to rising early so this is when there is the most to see and do. You will make the most of your trip if you succeed in adapting to their rhythm. If you are well rested, you will be better equipped to deal with all the new sights and experiences and you will have more energy to take part in the activities on offer.