Fatigue and sleepiness can be caused by many factors, such as:

• Poor diet: Eating too much or too little, or consuming foods that are low in nutrients and high in added sugars, fats, and processed ingredients can affect your energy levels and metabolism. ^{1 3}
• Excessive alcohol or drug use: Alcohol and drugs can interfere with your sleep quality, mood, mental clarity, and immune system. They can also cause dehydration, liver damage, and other health problems that can make you feel more tired and sluggish. ^{1 7}
• Stress and burnout: Chronic stress can take a toll on your physical and mental health, leading to fatigue, depression, anxiety, insomnia, and other symptoms. Stress can also trigger inflammation, hormonal imbalances, and immune system dysfunction, which can worsen fatigue. ^{1 7}
• Inactive (sedentary) lifestyle: Lack of physical activity can make you feel more tired and less motivated. Exercise can boost your energy, mood, blood circulation, and oxygen delivery to your cells. It can also improve your sleep quality, immune system, and brain function. ^{1 7}
• Jet lag: Traveling across different time zones can disrupt your circadian rhythm, which is your body's natural sleep-wake cycle. Jet lag can cause fatigue, insomnia, irritability, and difficulty concentrating. ^{1 7}
• Sleep disorders: Sleep disorders such as insomnia, sleep apnea, narcolepsy, and shift work sleep disorder can affect your sleep quality and quantity, making you feel tired and sleepy during the day. They can also increase your risk of other health problems, such as heart disease, diabetes, and depression. ^{6 7}
• Medical conditions: Some medical conditions can cause fatigue, such as anemia, diabetes, thyroid disorders, heart disease, kidney disease, liver disease, infections, autoimmune disorders, and cancer. These conditions can affect your blood, organs, hormones, or immune system, causing symptoms such as weakness, thirst, weight loss, pain, or swelling. ^{1 3 5 6}
• Psychological issues: Some psychological issues, such as depression, anxiety, and seasonal affective disorder, can cause or worsen fatigue and sleepiness. These mental health conditions can affect your mood, motivation, energy levels, and sleep quality. They can also interfere with your daily activities and relationships, making you feel more exhausted and overwhelmed. ^{1 2 5 6}